In the London Underground, a young couple discovers an unconscious man who turns out to be a prominent government official. They report the incident to the police, but when the official mysteriously disappears from the hospital, an investigation is launched. The case attracts the attention of Inspector Calhoun, a stubborn and cynical detective who begins delving into a series of missing persons reports. His inquiries lead him to suspect that something sinister is occurring within the tunnels of the Underground.

Calhoun's investigation uncovers a disturbing history involving workers trapped in a cave-in during the construction of the subway system decades ago. While it was believed that all perished, evidence suggests survivors may have descended into a primal existence underground, preying upon travelers. As Calhoun and the couple dig deeper into the unsettling mystery, they face perilous encounters and uncover horrific secrets hidden within the depths of the transit system. Their growing fear and confusion culminate in a suspenseful stride to unravel the truth behind the eerie vanishing acts.