The crew finds and reassembles Lore, an earlier android built by Data's creator, who plots to hand the Enterprise to the Crystalline Entity.
The Enterprise visits Omicron Theta, the colony where Data was found and which was destroyed by the Crystalline Entity. In the underground laboratory of Dr. Noonien Soong the crew finds the parts of a second android identical to Data, and reassembles him aboard ship. He calls himself Lore and describes himself as Data's brother.
Lore turns out to be an earlier, more humanlike model whose ambition made the colonists fear him. He disables Data, takes his place, and tries to deliver the ship to the Crystalline Entity, which he had lured to the colony before. Wesley Crusher grows suspicious but is not believed until Data recovers and confronts his brother.
In the end Wesley beams Lore out into space. Lore returns in later seasons, and the Crystalline Entity comes back in the fifth season episode Silicon Avatar.