Twenty-five years ago, the Keepers of the Keys – consisting of Rendell Locke, Kim Topher, Jeff Ellis, Mark Cho, Erin Voss, and Ellie Whedon, and her boyfriend Lucas Caravaggio – used one of several magical keys, the Omega Key, to open the Black Door, leading to the possession of Caravaggio by a mindless shadow demon. Tragically, Locke is forced to kill Caravaggio and the keepers somberly disband. Locke’s brother, Duncan, saw what happened, so they had no choice but to remove his memories and use the Plant Key to hide them. In the modern day, Ellie uses one of the keys to summon a mysterious entity named “Echo” – whom she nicknames “Dodge” – into the Wellhouse, assuming she was bringing Lucas back from the dead and unintentionally trapping Echo there, leaving the infuriated and emaciated girl consumed with revenge. Nine months later, a man phones Ellie from in an eerie town, fearing for the exposure of his memories to forces of darkness. Removing a key from his safe, he jabs it into his chin, causing himself and his entire house to explode into flames. Three months after that, Rendell is visited by underage criminal Sam Lesser, who sadistically murders him when he refuses to disclose information regarding Keyhouse, resulting in the Locke family – consisting of the outgoing Tyler, the introverted Kinsey, little Bode and their kindly mother Nina – relocating to their family home. They are welcomed there by their uncle Duncan. The following day, Tyler and Kinsey begin school while Duncan leaves to head back to Boston for work, and hands Keyhouses’ keys over to Nina. As a new member of the school hockey team, Tyler attends a party and meets Javi and Eden Hawkins, two extremely popular yet narcissistic Matheson students, and flirts with Eden’s close friend Jackie Veda. Kinsey meets up with Scot Cavendish and his informal friend group calling themselves the Savini Squad, and they watch a movie, but she leaves upon the flawed realization that she is not a heroine at all, much to Scot’s chagrin.
At home alone, Bode comes upon a locked outhouse containing a well and becomes privy to a conversation with the Echo, who is still in the well and remains unable to escape. She asks if he has found any of the magical keys hidden throughout Keyhouse Manor, disclosing their existence as she hopes to finally return them into the bigger picture against the late Rendell’s wishes. He finds the Anywhere Key in Kinsey’s bracelet and uses it to steal ice cream, proceeding to confront Kinsey, who disbelieves his story. Hearing whispers, Bode finds the Mirror Key in the garbage disposal, which Echo reveals that the key allows the user to commune with the dead; she tells him to use it with adult supervision. His attempt to see his father backfires horribly when Nina most unwisely steps into the mirror, only to find herself in an ominous dimension she cannot escape, leading Bode to consult Echo once more. Sardonically, albeit subtly, she inspires him to use the skills he will desperately require going forward to rescue Nina himself, wishes him luck, and proceeds to infuriate him by taking the Anywhere Key and leaving onto a rainy street after several months of incarceration. Kinsey suggests sending Tyler in with a rope, resulting in their mother’s successful rescue. Everyone questions what occurred, and Bode reveals the key and the magic around the house. However, Nina has no memory of the incident, confusing her children, but Bode is later able to prove to Kinsey and Tyler that they had not been hallucinating. Mistakenly believing their misunderstanding about the Mirror Key was a malicious deception, Bode spends the next few days attempting to decode the situation surrounding the keys and the woman from the Wellhouse. In the mix, he bumps into Rufus, who explains that he looks after the grounds at Keyhouse, and the two bond over their shared affinity for action figures. Meanwhile, Echo ominously visits Sam, who was imprisoned after failing to kill Nina or Kinsey. Clearly famished and in dirty clothes thanks to her lengthy imprisonment, she proceeds to use the Anywhere Key to steal fast food and burgle numerous luxury items to wear. She has sex with a young man later that night until she energetically incapacitates him. In the late morning, she returns to Bode with a peace offering, only to deduce that he has taken a decoy key from a hardware store as bait to cripple her over a bear trap and blackmail her into returning the key, as had been Rufus’ suggestion earlier, and he proceeds to lie that he has not found any more keys. Feeling betrayed and infuriated, she tells him that when he finds the keys he is to give them to her.
Nina befriends Ellie Whedon, one of the former Keepers of the Keys, as well as Tyler’s English teacher, Joe Ridgeway. Later, Tyler, who has told his friends he enjoyed his affair with Eden (although they never had sex the previous night), hangs out with Javi and Brinker until they attempt to force him to steal from the convenience store. When half of Scot’s production cast leave due to issues over the script, Kinsey agrees to fill in, becoming an integral part of the production. However, she storms out upon experiencing flashbacks to the tragic event that brought them to Keyhouse, although Scot tells her everything is fine and they can re-film. Tensions rise between Tyler and Kinsey when Tyler returns home, and Kinsey reveals that she is angry that Tyler doesn’t talk to her anymore. The two argue until Kinsey notices Bode in his room with the key he found earlier inside the vacuum cleaner now in his neck. Tyler and Kinsey are in awe of the metaphysical box before them, and Bode reveals that they are inside his head, where feelings too come to life. Fearing the damage the keys could cause, Tyler and Kinsey take the keys from an irate Bode, holding them, with Kinsey suspecting Rendell must have known about them. Bode reveals the woman from the Wellhouse and her quest for the keys. In the meantime, Echo seeks out Mark and goes to a derelict building in search of a safe. She notices a key around a nearby child’s neck, prompting her to push him onto the railway track to take the relic from him and preserve their secrecy. Bode is drawn to a painting on the mantelpiece, and finds another key hidden within. Opening a large door in the library, he enters astral form and flies to the graveyard on Keyhouse estate, where he meets the ghost of Chamberlin Locke. During a hockey game, Tyler notices Kinsey being harassed and steps in, eventually causing a fistfight, and so Nina is spoken to about his behavior. She befriends Ellie, but the gym teacher gives off the impression that she knows more about Keyhouse than she is honest enough to let on. Tyler eventually apologizes to Kinsey about the fight the previous night, but Kinsey proceeds to use the Head Key on herself, leading into a metaphysical Mall. After probing some of her memories, they are attacked by a savage woman with blonde hair, but are able to fend her off. After a talk with her mother, Kinsey bails on going to the theatre to view a local cover-band with Scot, proceeding to use the Head Key again, dragging the figure seen previously and burying it, essentially killing her fear.
The first effects of Kinsey’s partial exorcism become prominent when she eagerly cooks breakfast, including Bode’s favorite food bacon, for the family. Ellie visits Nina at Keyhouse, and Nina asks if Ellie could help her get in touch with some of Rendell’s friends, only to learn none of them can be a help at all. Nevertheless, Ellie finds it therapeutic being back at Keyhouse, and shows her a game room full of old artifacts. Nina questions if Ellie is looking for something, but Ellie dismisses the idea. Nina spends the next few days reminiscing over times spent with Rendell, visibly upsetting her. After hesitantly agreeing to get involved in the 5K, Tyler questions Mr. Ridgway about the event, and he reveals that it is all part of trying something new. Returning to Keyhouse, he finds the Head Key and, in return for Bode’s help doing so, allows him to join him in his head; revealing that he wishes to put a history book in, much like Kinsey took something out. Tyler and Jackie hit up local businesses downtown after school and later bump into each other in the library, where Tyler is checking out one of Jane Austen’s books. The two go to dinner after deciding on which is the best restaurant. As planned, Scot visits Keyhouse with Kinsey, who gives Scot a tour and tells him about the keys, presenting the Head Key as evidence; the two share a kiss. Returning home, Tyler offers Kinsey food from Bill’s as a peace offering, and she accepts his apologies. The Echo visits Erin Voss, who has long been mute from a past accident, at the psychiatric hospital, revealing that the majority of her friends are now dead and threatening to penetrate her mind. Shortly thereafter, she ambushes Bode. He attempts to beat her with a stick, but she deflects his assault, takes him outside, and surrounds him in a ring of fire, only for him to realize she cannot take the key, which leaves her with no choice but to leave for the time being. Kinsey takes back the harsh words she had said previously, apologizing to her mother and the two embrace.
The first key not to be found by Bode manifests itself before Tyler and Kinsey inside a piano – the Music Box Key, capable of controlling the actions of another. At school, Kinsey shows it to her friends and, watching from afar, attempt to humiliate Eden Hawkins and avenge everyone she has bullied. When she charmingly passes it off as comedy, fellow Savini Squad member Gabe notices the situation and creatively launches a hilarious spectacle that eventually goes viral, making Jackie and Tyler furious. While Tyler becomes closer with Jackie and begins helping with the organization of a fundraiser and so never rejoins the hockey team, Bode’s school life, while not disclosed, is shown to consist of him getting in trouble for bringing a hammer to school, having intended to use it if Echo infiltrated the facility. Kinsey and Gabe walk through the woods, and the two speak of the keys until Gabe reveals that he had to transfer to Matheson because his parents are getting divorced. Nina visits Mr. Ridgeway at home for advice, wondering if something deeper is going on. At Jackie’s house, the two talk about Tyler’s life in Seattle and proceed to make out. At home, Scot asks if they can use the keys in secret to make their movie better, and Kinsey agrees. Both respective siblings are sidetracked when they are drawn by the whispers to the graveyard, where they find a key hidden in a vase. Seeing a keyhole on a nearby tree, they cause glowing jars to emerge from the floor, each being a memory, explaining why Duncan had mysteriously forgotten all of the manor’s lore. This makes them forget a heated argument that had resulted in Bode getting the Music Box Key.
At Matheson Station, Nina sits and awaits a meeting with the officers. She talks with Detective Mutuku, and attempts to tell him that Mr. Ridgeway wouldn’t have killed himself, marginally suspecting a murder, and having no idea Ellie Whedon had been present. Kinsey opens her bag to reveal one of the memories from the cemetery, attempting to refresh Duncan’s recollections. Although she does not succeed, Tyler becomes furious when one of the memories shows Rendell killing his friend and declares that he is done with the whole ordeal. In the mix, Nina suspiciously questions Ellie about having identical scars with Rendell, who is unable to form an efficient lie and so storms off in anger. Back at Keyhouse, Bode is drawn to to a sewing room where he finds another key, which he cannot figure out how to use. Deciding she has had enough of his antics, Echo approaches him in the back and threatens that he will be the downfall of his entire family – foreshadowing that Sam has escaped from prison by using the Matchstick Key given to him by Echo, who proceeds to escort him to Massachusetts by Anywhere Key. Having decided to film there upon losing access to the docks, Scot, Gabe, Kinsey and the rest of the film crew arrive at the Drowning Caves and are instantly mesmerized. Drawn further in, Kinsey finds a mysterious and ominous door with an omega symbol on its lock. When the tide rises, however, everyone narrowly escapes with their lives; Gabe bravely rescues Kinsey from the clutches of death. Kinsey bonds with Gabe over this and simultaneously falls out with Scot over losing much of their filming equipment. Tyler attends Joe’s remembrance party, but receives numerous messages from an infuriated Jackie, who reminds him that the 5K run is in progress, leading to a huge falling out. Tyler is in awe when Echo enters the party, believing her to be highly attractive – the two hit it off, meet together at the convenience store, and have sex in her car after she reveals her nickname as Dodge. However, Echo loses grip of the Anywhere Key and Tyler, presuming her to be the “Well Lady”, makes a narrow escape with it.
Meanwhile, as Duncan leaves the grounds of Keyhouse Manor, Sam Lesser walks onto the estate, consumed with revenge. Bode insists on keeping by Kinsey’s side, but are roused to action when Sam threatens Nina’s life. He holds the Locke Family hostage, now in possession of the Mirror Key, Music Box Key, and Plant Key. He interrogates them for the Head Key, which is hidden in a plush toy, unwittingly revealing the existence of the keys to Nina. Several flashbacks show Sam acting in malicious and childish ways and refusing help from Rendell and reluctantly accepting it from Dodge when she communes with him. Arriving home, Tyler battles Sam until he overpowers him and forces them into the woods, returning empty-handed just as Echo arrives, by which time Nina has called the police. Sam refuses to honor his word and hand over the newly acquired Head Key, demanding she confirm she shares his childish and baseless malevolence, but deciding she’s had enough of him, she silences him with a knife, leading to his spirit becoming trapped on the property. Flustered by everything that is going on, Nina begins drinking again, however, she wants to get ahead of the cycle and throw the alcohol away.
The following morning, everyone brings the Lockes comfort food as their condolences. Nina receives a visit from Detective Mutuku, who tells her that he just wants to check on her in an unofficial capacity and is reassured the Lockes are getting back on track. In the meantime, Rufus and Ellie are revealed to have a mysterious guest, who is Lucas Caravaggio, Ellie’s childhood boyfriend. He is trying to force her to do something she doesn’t want to do. Tyler finally makes amends with Jackie. Having told Tyler earlier that the Echo plans to open the Black Door, Kinsey leads him there personally, where she apologizes for removing her fear and nearly getting her friends killed. Arriving at Erin Voss’ ward, Kinsey points out a photograph with Lucas in it to Erin, who makes it clear that Dodge and Lucas are one and the same. Meanwhile, Nina finally finds a key of her own, the Mending Key, failing to use it to revive her husband from the dead. Kinsey reveals that they have tried to reveal the existence of magic before, however, Nina was unable to remember. Kinsey begs Nina not to turn to drinking as the Locke Children need her. Tyler finds the Omega Key, and he and Kinsey are in awe to realize they have found just what Dodge is looking for.
A flashback to three months prior shows Ellie Whedon fetching a crown in Keyhouse Manor for the Echo (disguised as Lucas Caravaggio), but then Joe Ridgeway stalks them with his camera, making Echo furious; it is then revealed that in retaliation, it was she who had killed him. In the present day, Echo deduces that the Omega Key lies with Rendell’s remains, as she had recently used the Head Key on Erin Voss. Bode finds the Matchstick Key in the yard and tells Rufus of Lucas’ real identity, as he has already been let in on the secret when they had been working on model airplanes together. Eden approaches Kinsey, angry over having seen a creature – who is actually Kinsey’s fear – jump out on her that morning. Kinsey makes it clear she is not responsible when she discloses the existence of her fear to Eden while Tyler reveals the existence of the keys and Dodge to his friends, all in the hopes of being united when they go up against Dodge. Kinsey naively questions if she can date both Scot and Gabe, only to receive a call from Ellie, who asks if the two can meet at Keyhouse. Ellie had recently revealed to Rufus that they were leaving town, and that while she was doing what she thought was right, she has had a change of heart. She reveals the events that separated the Keepers of the Keys twenty-five years past, using the Head Key to illustrate her story. Echo has an extremely mysterious key of her own, which Ellie had used to summon her into the Wellhouse, assuming she was bringing Lucas back from the dead and unintentionally trapping Echo there, and she has sought revenge ever since. Bode gives Ellie the Shadow Key, and Ellie leaves to retrieve the Crown of Shadows from her house, but Echo takes it from her and returns to Keyhouse Manor, using the Shadow Key to unlock and harness its powers and command semi-sentient shadow demons from the Black Door. The demons storm the manor until Bode attaches the Matchstick Key to one of his toys and incapacitates a decoy of Dodge. With the trio being none the wiser, they ultimately decide to put Echo in the Black Door, calling backup in the form of the Savini Squad for the extraction to the Drowning Caves.
With the Echo defeated, Scot and Kinsey call it quits on their relationship, but promise to remain friends. Bode, meanwhile, bikes to the Whedon house to check on Rufus, who is still unconscious, and Ellie, who is missing. He calls the police, forcing Detective Mutuku to end the coffee date he has been having with Nina. Inside the ambulance, Rufus gives Bode the Head Key, prompting him to return home for verification that the Echo is gone. The next morning, Nina tells the children that she made a mistake moving them to Keyhouse Manor, and now they are going to move back to Seattle if they sell the house. The children are not happy with the announcement, however, and tell Nina that they like Matheson and do not want to move back to Seattle. Bode writes a letter to Rufus, telling him that the keys were fun at first, but are now a part of who they are and they must protect them. The three Locke children, the four Savini Squad members, and Rufus have become the second generation of the Keepers of the Keys. The Locke Family takes a trip to the cliffs above the Drowning Caves to spread Rendell’s ashes into the wind. Although everything seems to have ended well for the Lockes, a series of flashbacks reveal that a second decoy had been made when Dodge used the Identity Key on Ellie, triumphantly finishing her quest for revenge, and that Eden was hit by one of the shadow demons when the Black Door was open previously. It is revealed that all along, Gabe has been Echo, as Echo meets with the now possessed Eden.