The Changeling plot uncovered in 2401, in which renegade shapeshifters had replaced Starfleet officers ahead of their joint attack with the Borg on Frontier Day.
After the Dominion War a renegade faction of Changelings held a lasting grudge against the Federation. Some of them, including their leader Vadic, had been held and experimented on by Section 31. By 2401 they had learned to copy the internal organs of the people they imitated, so the usual blood screenings could not catch them.
Using this, the Changelings replaced officers throughout Starfleet. Raffi Musiker of Starfleet Intelligence and her handler Worf followed the trail after the theft of a portal weapon, and Ro Laren found that the infiltration reached deep into Starfleet's command.
The Changelings were working with the remnants of the Borg. They stole Jean-Luc Picard's original human body so the Borg could use the altered genetic code it contained. That code had been slipped into Starfleet's transporters and was meant to let the Borg take over young officers across the fleet. The plan came to a head in the Frontier Day attack, which Picard and his old crew defeated.