Chloe, a struggling artist and former drug mule, gets pulled into doing one final job: Running drugs across London for an old contact, the elusive Mads. Carrying a holdall of narcotics and an envelope of cash, Chloe boards the last train on the London Underground, only to find herself trapped in a nightmare. A masked killer slaughters her fellow passengers in a blood-soaked frenzy, and Chloe barely escapes, fleeing into a labyrinth of tunnels. Captured by this grotesquely deformed psychopath, she’s thrown into a makeshift prison with other terrified captives. With the help of sharp-witted rapper Natter G and the resourceful Nina, she leads a desperate fight through a nightmarish maze.