The story unfolds across two parallel threads:
1. The Jimmies: Spike is forced into a fight-to-the-death initiation, killing "Jimmy Shite" to survive. Renamed "Jimmy," Spike witnesses the cult’s brutal raids, led by Jimmy Crystal and his trusted lieutenant Jimmy Ink (Kelly). Crystal enforces strict ritualistic conformity, forcing everyone to wear tracksuits and worship his inverted cross iconography.
2. The Bone Temple: Dr. Ian Kelson maintains an ossuary shrine made from skulls. He captures Samson, an Alpha Infected, and conducts trials with targeted neurological sedatives. Kelson realizes the Rage Virus overloads the brain's adrenal systems; by administering precisely timed dosages, Samson's hyper-aggression recedes, revealing coherent human thought and childhood memories.
The Convergence & Ending: Jimmy Crystal’s gang discovers Dr. Kelson's Bone Temple and attacks it. During the siege, Kelson realizes his research must survive. He shields Spike and uses his final moments to administer a permanent cure compound into Samson’s bloodstream. Kelson is mortally wounded by cult members, but dies knowing his medical theory was correct.
In a shocking turn, Jimmy Ink exposes her real name as Kelly. She turns on the megalomaniacal Jimmy Crystal, incapacitating him. Kelly and Spike crucify Jimmy Crystal upside down on his own inverted cross at the Bone Temple, leaving him helpless as infected approach.
Kelly and Spike escape together. Samson, now completely free of viral rage and fully restored to human consciousness, quietly wanders into the wilderness—a living proof that the apocalypse can be cured.
Epilogue: Spike and Kelly flee across the countryside, pursued by wild infected. They stumble upon a secluded farm cottage, where an older Jim (Cillian Murphy) is living with his daughter. Jim steps outside with his weapon, rescuing Spike and Kelly and taking them into his home. This sets the stage for the final chapter of the trilogy.