When the crew beams aboard a deactivated robot, Torres works night and day to revive the mysterious mechanical being. After exploring many dead ends, the Chief Engineer finally finds an appropriate power source and is delighted when the sentient artificial lifeform is reactivated.

The robot introduces itself as Automated Personnel Unit 3947, one of a nearly extinct line of workers created by the Pralor, a species of humanoids who no longer exist. It asks Torres to build a prototype power module for the construction of additional units, but Janeway points out that this would be a violation of the Prime Directive. However, the unit won't take no for an answer; when they return 3947 to its vessel, it renders Torres unconscious and takes her to the Pralor ship, where others of its kind are waiting.

To Janeway's dismay, the crew is unable to penetrate a subspace defense shield that goes up around the Pralor vessel, and Torres' combadge is deactivated by the robots. When Voyager fires on the alien ship, the robots respond by launching a violent attack that threatens to destroy the starship. To halt the assault, Torres finally agrees to build the desired prototype.

While the Voyager crew plots to rescue her, Torres learns that the robots have been unable to produce their own prototype because each power module has an individual energy code. She sets out to design a standardized module that can power any unit.

Another ship approaches, manned by similar robots. The second alien vessel begins firing on the first. Completing her work, Torres finds out that the builders of these robots, the Cravics, used the machines to fight their war against the Pralor builders. All of the warring robots were programmed for victory, and when the Pralor and Cravic humanoids decided to call a truce, the robots terminated them and continued their battle. The new prototype that Torres has created will allow the Pralor robots to win their war against the Cravic robots. Horrified, Torres destroys the prototype, and the crew is able to beam her away from the Pralor ship, leaving the robots to continue their war.