Immediately, strange things begin to happen on the U.S.S. Enterprise. It seems that both Lt. Worf and Dr. Crusher have become afflicted with confusing behavior, while the ship malfunctions and repairs itself with no available explanation.
Soon after entering the strange "cloud," Assistant Chief Engineer Singh is killed while investigating the ship's mechanical malfunctions. At once the Starfleet crew begins investigating the circumstances surrounding Singh's death. Tasha Yar questions the visitors aboard the Enterprise and Counselor Troi puts Lt. Worf and Dr. Crusher under hypnosis to determine their thoughts during their spells of strange behavior. Both Dr. Crusher and Lt. Worf reveal that "it felt as if someone or something had invaded their minds and bodies."
After coming into contact with this strange being, Captain Picard shocks the crew with his bizarre actions. He commands the U.S.S. Enterprise back into the "cloud," thwarting the original mission to deliver the aliens to Parliament. The Captain, still under the being's influence, announces that in its struggle to return home, the entity accidentally disabled the ship, killing Singh and invading Lt. Worf, Dr. Crusher and himself. The being, now in full control of Captain Picard's behavior, announces Picard's resignation. Despite the crew's objections and their determination to fight the force that seems to be taking over Picard's mind, their efforts are in vain. The Captain is gone.
Commander Riker takes command of the U.S.S. Enterprise and orders the ship to proceed towards Parliament. By this time, the real Captain (in the form of energy) manages to move into the transporter relays and materialize back aboard the starship. (-Courtesey of Startrek.com)