The 2401 Borg and changeling plot that turned Starfleet's young crews against Earth on Frontier Day, stopped by Picard and the old Enterprise-D crew.
Frontier Day was Starfleet's annual celebration of the launch of Archer's Enterprise NX-01. In 2401 the whole fleet gathered near Earth for it, and the Borg Queen chose that moment to strike, working with rogue changelings who had stolen Picard's original human body and used it to hide Borg genetic code in Starfleet's transporter systems.
Everyone who had passed through the altered transporters was changed, and on Frontier Day the code took control of Starfleet's younger officers. The fleet, now linked through the new fleet formation system, opened fire on Earth and on the older officers. Jack Crusher, Picard's son, who carried Borg-altered biology from Picard's own past, was used by the Queen as her voice.
Picard, Riker, Worf, Data, La Forge, Troi and Crusher took the restored Enterprise-D from the fleet museum and found the Borg cube hidden near Jupiter. Picard reached Jack and persuaded him to break free, and the cube was destroyed, freeing the fleet. Seven of Nine and Raffi Musiker also retook the Titan-A during the crisis.