Geordi is assigned to survey the station core of a new mining development with Dr. Farallon, the scientist who is developing the technology. When a dangerous problem suddenly occurs in the power grid, Farallon chooses the moment to introduce Geordi to an exocomp, a device she constructed that quickly restores the power grid to its normal state. Later, Farallon explains the intricacies of the exocomp to Data, pointing out that the device not only replicates tools, but can also solve a myriad of problems. This convinces Picard to allow Farallon to use the exocomp to repair the mining system, with Data assisting her. The device initially performs its tasks without error, but suddenly becomes unresponsive to commands just before an explosion occurs in the plasma conduit
Back on the Enterprise, Farallon explains that the exocomp has malfunctioned and is unusable. She and Geordi choose to continue their work with two other exocomps, while Data, who is intrigued by his fellow machine, takes the faulty item back to his quarters. He is surprised when, a short time later, the exocomp repairs itself. Just as Geordi and Farallon are about to leave, he rushes in and tells them they must stop — because the exocomps are alive.
During a meeting with the staff, Data explains that the exocomp shut itself down just before the explosion in the plasma conduit, indicating that the machines are capable of self-protection. The staff tests Data's hypothesis by putting a device in simulated danger, and Data appears to be proven wrong when the exocomp completes its task. But Data remains unconvinced, and after repeating the experiment several times, discovers that the exocomp knew the test was only a simulation and acted accordingly.
Picard, Geordi and Farallon return to work at the station core, but soon learn they've lost internal confinement and are risking exposure to dangerous radiation. Farallon and several workers are able to transport back to the Enterprise, but Geordi and Picard are left behind. With no other solution, Riker asks Farallon to transport the exocomps to the station to save the men. She agrees, but Data refuses to allow her to do so and locks out the transporter controls.
Claiming he can't justify sacrificing one life to save another, Data holds firm. Riker is able to convince him to compromise by allowing the exocomps to "choose" whether or not to risk their lives. To everyone's surprise, the exocomps come up with their own plan that saves Picard and Geordi and only sacrifices one of the three machines. The crew is left to ponder what they have witnessed, while Data is pleased that he could act as an advocate for these forgotten "living" things.