The film begins with photographer Adam Faulkner (Leigh Whannell) waking up in a bathtub filled with water. In his instinctive flailing, his foot catches and removes its plug; as the water drains a glowing blue object can be briefly seen to be washed away with it. After a few cries for help it is revealed that he is not, in fact, alone. Surgeon Dr. Lawrence Gordon (Cary Elwes) is on the other side of the same room, and soon finds the switch to turn on the lights.
Both men are inside a grimy, dilapidated industrial bathroom, chained to pipes at opposite corners of the room. Between them, out of their reach, is a body lying in a pool of blood, holding a revolver and a microcassette recorder. Both men discover envelopes in their pockets which contain microtapes; Gordon also holds a bullet and a key that does not unlock their shackles. Adam, with Lawrence's help, manages to snag the player from the body with which they play their tapes. Both tapes have the same voice, distorted by a pitch modulator. Adam's tape refers to him as a voyeur and asks, "Are you going to watch yourself die, or do something about it?" Gordon's tape reveals he must kill Adam before six o'clock (within seven hours as evident by a clock on the wall), or his wife and daughter will die and he will be left in the bathroom, presumably forever to starve to death. "Let the game begin..." the voice concludes. Hacksaws are soon discovered in the toilet tank; neither is sufficiently sharp to cut chain, and Adam accidentally snaps his in frustration. Dr. Gordon realizes that the saws are meant instead for their own ankles, which, if sawed through, would free them from their shackles.