The episode begins with one young boy critically ill due to a prank pulled by his older brother. In the midst of rushing to a starbase for medical treatment, Data suddenly has a compulsion to take over the Enterprise and not respond to any of his shipmates.
Data takes over the bridge and Picard retreats to engineering. After re-routing transporter control, Data must leave the bridge in order to get to the surface of a planet to which he has been summoned. The crew watch helpless as Data transports down.
On the surface, Data meets his creator, Dr. Noonien Soong. We learn that Data was created in Dr. Soong's own image. Initially disappointed with Data's entry into Starfleet, Dr. Soong eventually accepts his creation for who he is. Suddenly, Data's "evil" brother Lore enters the room having been rescued from the vacuum of space by another ship.
Data discovers that Lore lied to him at their first encounter. Data was created as a more improved version of Lore. At this point, Data warns Soong that Lore does not have the best intentions here.
Lore disables Data while Dr. Soong is asleep and tricks Soong into installing a chip in his brain that was meant for Data. Lore knocks over Soong and fatally injures him.
Back on the ship, a remorseful Data gives control back to Picard and the two young brothers in sickbay are doing better and healed. Data can't seem to understand why the two are now getting along. Dr. Crusher says, "They're brothers, Data. Brothers forgive."