Graves' young assistant, Kareen Brianon, has summoned for help against Graves' wishes.
An away team, led by Data, beams down to assist the dying Graves. Data develops a particularly strong rapport with the brilliant scientist, who is bravely enduring the final stages of a terminal disease. Shortly after Graves tells Data of his remarkable ability to transfer human knowledge into a computer, he dies.
Back on board the Enterprise, Data displays extremely uncharacteristic behavior, going so far as to accuse Picard of having lustful intentions toward Kareen. After an engineering test reveals no mechanical malfunctions with him, Troi tests Data's psychological reactions and discovers two conflicting personalities within him — Data's normal personality, along with a brilliant but irrational side which is quickly taking over his mind.
Meanwhile, Data reveals to Kareen that he is actually Graves, who transferred his dying mind and body into Data's mechanical frame. He also tells her that he intends to place her in an android body so they can be together eternally. A frightened Kareen rejects the plan, which triggers a violent reaction in Graves/Data.
Having discerned Graves' scheme, Picard confronts Data and begs Graves to vacate Data's body and mind. In response, Data knocks the captain unconscious. But when Picard is revived, he is relieved to learn that Graves' knowledge has been transferred to the ship's computer, thus returning Data back to normal.