Six months after leaving Chicago, Father Tomas continues his training to become an exorcist under the watchful eye of his partner Marcus Keane. Their travels lead them to a troubled young woman in rural Montana, where their investigation puts them in the crosshairs of her extended family. But as Tomas' skills continue to grow, Marcus begins to worry that his apprentice is taking unnecessary risks. The action then switches to a secluded private island off the coast of Seattle, where Andy Kim runs a group home for at-risk foster children, whose lives are disrupted when the state sends a social worker to determine whether the foster home should be closed. Little do they know the true danger is yet to come: a malevolent presence that has set its sights upon one of the children in Andy's care.
Tomas and Marcus attempt to escape the custody of Cindy's enraged family, while simultaneously trying to finish her exorcism. Rose conducts her inspection of Andy Kim's household and is concerned about Caleb's mental state. Meanwhile, Andy's neighbors offer to teach the children farming skills, but there's something sinister behind their intentions. Additionally, Verity may be hiding something from the rest of her foster family. Then, Father Bennett tries to warn the Office of Exorcism of a new threat, but finds his attempts railroaded by menacing forces.
The situation at the foster home continues to deteriorate, as Andy tries to help his foster daughter, Grace overcome her agoraphobia. Meanwhile, Marcus and Tomas travel to Seattle where they encounter a new possession case, discovering a young girl who has been targeted by a force of pure evil. Father Bennett tracks down a rogue church operative named Mouse who is waging a one-woman crusade against the demonic conspiracy.
Andy's foster home receives a surprising new addition as he confronts the strange incidents occurring on the island. Marcus investigates a series of natural disturbances with the help of a local wildlife biologist, while Tomas continues to question the strange visions that led him to Seattle. Meanwhile, Bennett's uneasy partnership with the enigmatic Mouse takes a dark turn.
As Tomas and Marcus continue to investigate the strange incidents at the foster home, the demon finally makes its presence known. The two priests must race against the clock to find evidence of possession before it is too late. Meanwhile, Andy and Rose take the children on an overnight camping trip, setting them on a collision course with the supernatural presence on the island.
A visit to the foster home carries deadly consequences, as Marcus and Father Tomas attempt to draw the demon out of hiding. Rose finds herself the target of the demon's wrath. And no one suspects that a new danger may be lurking in the woods, waiting for its moment to strike. Meanwhile, Father Bennett and Mouse take the fight directly to the conspiracy as they attempt to hunt down an old adversary.
As the exorcism begins, the fate of the foster home hangs in the balance. This is an adversary unlike anything Marcus and Tomas have ever faced, and in order to save this family, the two of them must journey into the mouth of madness itself.
As Father Bennett and Mouse head west to rendezvous with Marcus and Tomas, new details about Mouse's tragic past comes to light. With time running out to finish the exorcism, one of the foster children is faced with a difficult choice. But the demon has other plans.
As the exorcism reaches a critical juncture, Tomas engages the demon in a desperate battle to save Andy's soul. Meanwhile, Marcus and Rose attempt to get the foster children off the island, but the demon has other plans. As the storm of the century descends upon the island, salvation may come from an unlikely source.
Everything has led to this moment. As one priest fights for his life in the hospital, another is forced to confront the ghosts of his past. With time running out to save Andy's life, one of the exorcists must make the ultimate sacrifice.