Picard meets secretly with three other captains on a desolate planet where they urgently tell Picard that there is a vast conspiracy amongst the top brass of Starfleet. Picard doesn't believe it until shortly after the meeting, the USS Horatio is blown up... but not from an enemy ship. Their engines were detonated from the inside.
At Picard's orders, Data reviews orders and ship deployments over the past six months. He reports to Picard that he suspects that orders came down in preparation for an invasion into Federation space. Picard orders the ship to Earth immediately.
On Earth, the Admirals he meets are acting strangely. One comes on board and physically manhandles three men before he is stunned by Dr. Crusher. While sedated in sickbay, the doctor discovers an alien living inside the admiral and controlling his actions.
Discovered just in time, Picard and his first officer Riker contain the threat as they kill the "queen" of a hive-like race that inhabits and feeds off humanoid bodies. But an ominous future may be on the horizon as a homing beacon was sent off before the queen was discovered.