Alex just wants to get home. He's tired and aching from a long, stressful day, and the next carriage on the hot, noisy and busy London tube train he's travelling on looks enticingly quiet and calm. So why are so many of his fellow passengers avoiding it?

Regan has been having a rough time; her fiance has been cheating on her, her closest friend has betrayed her and she's lost her job. One evening, fed up of staying home and drowning her sorrows, she decides to go for an impromptu run. This decision results in her fighting for her life, in a nightmarish scenario she never could have imagined.