The Enterprise visits Omicron Theta, the colony where Data was found 26 years earlier, and investigates the mystery surrounding the colony's disappearance. They find the world to be drained of all life forms. They also find the living quarters and laboratories of the colony's residents, now all empty — as well as a disassembled android virtually identical to Data, and proof that famed scientist Dr. Noonien Soong was among the colonists and constructed these androids while living there.
"Datalore" is particularly worthwhile for the valuable and definitive backstory it offers regarding Data's origins. The story even cites Isaac Asimov in its dialog explaining Soong's dream to realize the "positronic brain." When the crew decides to assemble and activate the other android, they discover he is actually Data's "brother," Lore. He's an intriguing individual — similar to Data but different in many key ways — and the plot's progress gradually reveals Lore's penchant for villainy and his relationship with the crystalline entity, which feeds purely on life. Lore helped "feed" it the Federation colony all those years ago. Brent Spiner memorably creates two distinct individuals within the confines of similar android templates. Lore's villainy is intriguing right alongside Data's equally intriguing loyalty to Picard and Starfleet.
What hurts the episode, however, is that no one is smart enough to realize the obvious. None of the crew except Boy Wesley realizes that Lore is impersonating Data — and when Wesley (who is always somehow more observant than the silly adults) brings this to their attention, we get helpful lines like, "Shut up, Wesley," which lead to even more helpful lines of would-be teenage rebellion when Wesley complains about the officers not listening to him. Quite simply, this is annoying material, continuing in season one's trend of Wesley's child status as a plot device. Meanwhile, all the adults look terminally clueless.
Still, the episode shows more promise (and is more memorable) than many season one tales, with a battle of dialog and wits between Data and Lore, which culminates in a physical fight and Lore being beamed into space.