A man, Ryjkin, is trying to escape from the titular Cube. Upon entering a trapped room, he is sprayed with a mysterious liquid that he fears is acid only to find it sweet tasting, thinking it is flavored water, desperately drinking it to quench his thirst. However he discovers to his horror it is a potent molecular base that melts his entire body after a few minutes, killing him.
The rooms in the Cube are being monitored from a remote observation room by two technicians, Eric Wynn and Dodd, who are unaware of who their employers are. The pair regularly play chess during work, whereupon Wynn demonstrates mental calculator abilities which he uses to predict Dodd’s moves.
Wynn and Dodd are ordered to record the dream of a subject, Cassandra Rains. In her dream, Wynn sees Rains captured while walking in a forest with her daughter. Rains wakes up in the Cube and meets the other occupants: Robert Haskell, Jellico, Meyerhold and Bartok. Haskell has the same tattoo on his forehead as the soldier who captured Rains. However, Haskell, like everyone else, only knows his own name and has no recollection of his former life or how he got there.
According to what Wynn and Dodd know, everyone in the Cube faced a death sentence and volunteered to partake in psychological experiments instead. Rains’ consent form, however, is not found in her file. The captives venture through the Cube, testing each room for traps by throwing a boot in first. Bartok, Jellico and Meyerhold are all killed by various traps, leaving Rains and Haskell as the sole survivors.