On stardate 41242.4, the USS Enterprise stops by the planet Omicron Theta while on her way to Starbase Armus IX for computer maintenance. The planet is the site of a farming colony that had mysteriously vanished and also where the Tripoli originally found the android Data. On the planet, an away team finds that what was reported as farmland now looks like barren rock with no trace of life in the soil. Amid other indicators that something terrible has happened to the colony, the team also finds the lab that Dr. Noonien Soong, a formerly prominent robotics designer, built Data. Data finds various android parts and takes them back to the Enterprise.
As the course to Armus is resumed, Data's "brother" is reactivated in sickbay. Referring to himself as "Lore", he first tells others that Data was built first and he himself is the more perfect model. Later, in private, he tells Data that it was actually the reverse order, and that colonists became envious of his own perfection. He also explains that a crystalline space entity capable of stripping away all life force from a world was responsible for the colony's demise. Lore then incapacitates Data, revealing that he plans to offer the ship's crew to the Crystalline entity. When a signal transmission is detected from Data's quarters, Wesley arrives to investigate. He finds Lore (now impersonating Data) who explains that he had to incapacitate his brother after being attacked. Wesley is doubtful, but pretends to accept the explanation.
Soon after, the same crystalline entity that had attacked the colony approaches the ship. Lore, still pretending to be Data, enters the bridge as the object hovers before the Enterprise and explains that he used a shut off switch to incapacitate his brother, causing Beverly to be suspicious, since Data was careful about who knew of his off switch. Lore then explains that he can communicate with the crystalline entity and suggests that Picard show a demonstration of force by beaming an object toward the crystal and then destroying it with a phaser.
On the way to the cargo transporter, Lore catches Worf off guard and winds him with a powerful punch to the chest. The suspicious Beverly and Wesley reactivate the unconscious android and discover it is, indeed, Data. The three race to the cargo hold to find Lore telling the entity that the shields will drop momentarily when the transporter is activated. Data quickly rushes Lore and a brawl ensues. Wesley runs to the transporter controls and, just when Data manages to knock Lore onto the transporter pad, Wesley activates the beam and sends Lore into space. With Lore gone, the crystalline entity flies away.