A man named Alderson (Julian Richings) wakes up in a cubical white room with a hatch door in the center of every wall, the floor, and the ceiling; 6 doors, one on each side. He opens every door in the room and discovers that every room next to the room he is in is identical, except that they are different colored. After a little thinking, he decides to proceed to the next room. He enters the room-colored orange. As he tries to move, he is suddenly cut to pieces by a razor-sharp metal grate that swings down from the ceiling and returns back up.In another room, colored white, Quentin (Maurice Dean Wint), Worth (David Hewlett), Holloway (Nicky Guadagni), Rennes (Wayne Robson), and Leaven (Nicole DeBoer) meet. None of them know where they are, how they got there, or why they are there, so they decide to try to find a way out together. Rennes and Quentin have already discovered that some rooms contain traps. Rennes assumes that they are triggered by motion detectors, so he tests each room by throwing a boot into it first. As the group moves along, Leaven notices numbers inscribed in the hatchways between rooms, and Rennes throws a boot in another room, but it doesn't trigger any trap. He enters it, but he suddenly dies from a spray of acid to his face. The others realize that they must find a better way to test the rooms, because some detectors are only working on human beings.Quentin believes that each of them has a purpose to fulfill within the Cube: he is a police officer, who works with logical thoughts, Leaven is a mathematics student, Holloway a doctor and a conspiracy theorist, and she believes that the government is responsible for the building of The Cube. Worth, the most pessimistic of the group, is reluctant to talk about his background in his past life. Leaven theorizes that any room marked with a prime number is a room triggered with a trap. They soon find another person in the Cube, a mentally challenged man named Kazan (Andrew Miller), and Holloway insists that they bring him along.Quentin enters a room without prime numbers but is injured in its razor-wire trap, disproving Leaven's original theory about prime numbers. Tensions rise within the group, with Quentin becoming annoyed by Holloway's paranoia, Kazan's mental state, and Worth's reticence. Worth inadvertently reveals that he helped to design the Cube's outer shell but insists that he knows nothing beyond the technical details of the project. His information allows Leaven to determine that the numbers could indicate the Cartesian coordinates of the rooms. The group starts moving toward the nearest edge.They arrive close at the edge, but they then realize that every room over their room is triggered with a trap, and that they would have to return back and find another way. They test the traps with the rest of their boots and find a blue colored room which motion detector is activated with a sound. They make it their way across the room, but when Quentin passes the room, Kazan makes a sound and Quentin almost dies of blades coming out of the walls. They arrive at an edge room and find a gap between it and the outer shell. Holloway swings out to investigate, using a rope made of their clothes, but nearly falls when the Cube vibrates; Quentin catches her, then drops her to her death, telling the others that she slipped. As they rest, Quentin tries to persuade Leaven to come with him instead of the others. Worth and Kazan stop Quentin, who beats Worth and then throws him through the floor hatch. Rennes' body is down here, making them believe they have been going in circles, but Leaven and Worth quickly realize that the rooms have been shifting within the Cube - the source of the vibrations they have been feeling.Leaven deduces that traps aren't actually prime numbers but powers of prime numbers, and they are surprised when they discover that Kazan is actually an autistic savant who can quickly do prime factorization - to guide them. In addition, Leaven and Worth realize that there must be a "bridge" room that connects to the outer shell, and that the numbers indicate the positions that each room will reach as it cycles through the Cube. The room that connects to the bridge proves to be the one in which everyone, but Kazan and Alderson first met.Quentin goes insane and attacks the others. Worth incapacitates Quentin, who has now gone completely insane, buying time for himself, Leaven, and Kazan to reach the bridge. As Worth tells Leaven of his decision not to leave, since he has nothing to live for, Quentin returns and fatally stabs them both with a door handle. With the last of his strength, Worth drags Quentin back from Kazan so that he is crushed to death in the hatchway when the rooms shift next. Kazan, the only survivor, slowly exits into the bright light outside the Cube.