The Enterprise comes to the aid of terminally ill scientist Dr. Ira Graves (W. Morgan Sheppard), in the hopes of documenting his as-yet-unrevealed scientific discoveries before he dies. Graves, however, takes a very specific interest in Data and spends his final hours with the android. Data subsequently begins exhibiting strange behavior, the most amusing of which is the delivery of a ridiculous and indulgent eulogy for the recently departed Graves ("To know him was to love him, and to love him was to know him").
The plot is obvious to us, but not to the Enterprise crew: Graves, utilizing his own scientific breakthrough of combining the human brain and computer data storage, has transferred his consciousness and knowledge into Data and is vying for total control of Data's mind. The crew slowly begins to realize that Graves has somehow hijacked Data's personality. One major clue might be Graves'/Data's verbally expressed jealousy concerning Graves' assistant Kareen (Barbara Alyn Woods), and the ever-increasing size of Data's ego, which, by definition, should be nonexistent. I was amused by much of the Data/Picard interaction: Watching Data's sly insubordination and condescension toward Picard is a source of much of the episode's fun.
The episode's Serious Human Theme is whether this man Graves can retain his humanity now that he has superior android strength and mental abilities. And can he plausibly love Kareen, whom he previously admired without revealing his feelings on the account of their age difference? The other question is about Data's rights as a person, which Graves has usurped by hijacking his body. I like that the episode ends with a battle of reasoning between Graves and Picard, and that Graves proves Picard's point and is smart enough to fully realize that what he's doing won't work. But overall this is sort of an obvious storyline, and one that doesn't exploit its themes for what they're worth.