Aneurin Barnard plays Tommy, who, with his pregnant wife Amy Shiels Joanne, is moving out of their flat in a condemned tower block. In the lift, Joanne is cornered by a group of hooded teenagers. Tommy watches helplessly as the doors jam shut, and when he finally reaches her, he finds her beaten, with a syringe in her stomach. She is rushed to hospital, where doctors save the baby, but Joanne remains in a coma.
Eight months later, Tommy lives in a run-down council estate, caring for his baby daughter Elsa. The attack leaves him with a crippling fear of leaving the house, a condition known as agoraphobia. He visits Joanne at the hospice, but there is no hope of recovery. The compassionate nurse Wunmi Mosaku consoles him as he agrees to have his wife taken off life support, a grief-stricken decision that weighs heavily on him.
At Joanne's funeral, he encounters a foul-mouthed and eccentric priest who warns that the gang will return for his daughter. The priest is in the care of a young boy, Jake Wilson, who is blind but—strangely—appears to be able to "see" Tommy.
With redevelopment planned, the estate is emptied and Tommy is one of the last tenants. That night, hoodies break into his house and ransack it, apparently searching for Elsa. He calls Marie, but the line is cut before he can explain. In the morning, Marie finds him barricaded in the bathroom with the baby.
Marie invites them to stay with her for a few days. She tempers Tommy's fear by suggesting that the attacks could be random acts of violence by different gangs, but she remains sympathetic toward the youths. He asks to see the priest again, and Marie offers to help.
The priest reveals the tower block as a nesting place for creatures that are no longer human. He shares an urban myth: a pregnant junkie gave birth to twins who grew up within the block, turning feral and blind. They "see" people by sensing fear and often kidnap children. Danny, the young boy, was such a child, but the priest rescued him before he transformed. Although blind, Danny can sense fear and helps keep others hidden from the ferals.
Overwhelmed, Tommy refuses to join the plan, but Marie continues to support him. The next day, their bus route takes them through a long, poorly lit underpass. They encounter a group of hoodies; Marie bravely approaches them to pacify the situation, but she is killed.
Tommy flees with Elsa, while the ferals pursue. He begs the priest to intervene, and the priest agrees only if Tommy helps blow up the abandoned tower block. They travel to the building with Danny, who uses his ability to mask them from detection. A trap unfolds, and the ferals descend upon them, but the priest sacrifices himself to save Tommy and Danny. Before dying, he reveals himself as the father of the original infected kids. Tommy and Danny reach Elsa in the basement, but other turned children lie in cages beyond help.
Tommy and Elsa make their way toward the exit, only to be blocked by the ferals. Danny, who had warned he might not be able to protect everyone, is relieved when Tommy—having faced his fears—guides them through the crowd, and the creatures no longer sense their presence.
Outside, Tommy watches as explosions erupt, destroying the tower block and its mutant inhabitants, while he and Elsa begin a new, uncertain chapter away from the danger that defined so much of their lives.
