Dominion

Picard believes that the Changelings want to create a perfect replica of himself to target Starfleet's upcoming celebrations for the 250th Frontier Day. Geordi attempts to maintain Data's control of the synthetic body as it is overcome by the devious Lore. Picard devises a plan that traps Vadic and her crew on the Titan with force fields, using Jack—who is developing telepathy—as bait. Picard and Beverly interrogate Vadic and she reveals that she is hunting Jack for someone else. She also explains that she was one of ten Changelings whom Starfleet doctors experimented on to develop a breed of super-spies. Upon their escape, she swore revenge and formed the rogue Changeling faction, taking her torturer's form. Beverly confirms these facts, and Picard decides that he has to kill Vadic.

Summary from Wikipedia, CC BY-SA 4.0.