Imposters

Riker returns command of the Titan to Shaw, who contacts Starfleet. The USS Intrepid arrives to arrest Picard and Riker, led by Commander Ro Laren who betrayed Starfleet and Picard years earlier but has since returned from imprisonment. Ro forces Picard into the holodeck where she reveals her suspicions that the Changeling conspiracy has compromised Starfleet's highest levels. Picard reconciles with Ro and she gives him her treasured Bajoran earring. As Ro returns to the Intrepid via shuttle, she discovers a bomb planted by Changeling saboteurs and decides to fly into the Intrepid, sacrificing herself and damaging that ship. The saboteurs attempt to transport Jack off the Titan but his visions influence him into killing them. Picard and Riker realize the earring contains the intelligence Ro gathered on the Changelings and is also a communicator.

Summary from Wikipedia, CC BY-SA 4.0.