S1E7 · Lonely Among Us

The Enterprise is en route to the planet "Parliament" with delegates from two warring planets in the Beta Renner system, the reptilian Selay and the canine Antican, when the ship encounters a strange energy cloud. Unseen by the crew, Lt. Worf (Michael Dorn) is hit with a strange energy discharge as the ship passes the cloud, causing him to become violent. Doctor Beverly Crusher (Gates McFadden) sedates Worf and brings him to the sick bay, but is also infused with the energy as she examines his body. Crusher begins to act oddly, although not violently, to those around her and goes to the bridge, asking questions about the ship's navigational functions. When she questions Lt. Cdr. Data (Brent Spiner) at one of the science stations, the energy sparks between her and the console, leaving her confused as to why she is on the bridge.

The ship suddenly begins to malfunction and Captain Picard sends Assistant Engineer Singh (Kavi Raz) to investigate the cause. Singh is later found dead, after the energy goes into him, near a computer link, and Picard orders a murder investigation, considering the alien delegates to be prime suspects. Data investigates the murder in the manner of fictional detective Sherlock Holmes, and determines that the delegates were not responsible. Meanwhile, Counselor Deanna Troi (Marina Sirtis) uses hypnosis on both Worf and Crusher, finding that both recall being invaded by some entity during their blackouts. The ship suddenly drops out of warp for a second time and as Picard investigates the readouts at a bridge console, the strange energy transfers into him. The bridge crew becomes suspicious of Picard's actions after noting that all Enterprise systems are back to normal and that Picard has ordered them to return to the cloud.

The senior officers attempt to plead with Picard to undergo a medical examination and to step down from command, but he refuses. When they return to the cloud, Picard announces that they had picked up an entity previously when they passed the cloud, and now Picard and the entity are one. Under its influence, Picard plans to transport his energy back into the cloud, and he shoots energy at the bridge crew when they try to stop him. The crew are unable to prevent Picard from beaming off the ship.

The crew spend hours trying to locate Picard to no avail, so they are forced to accept he is beyond recovery and prepare to leave. However, Troi senses the Captain's essence nearby, and Picard manages to signal the crew through the ship's computers. Data is able to reverse the transport, reconstituting Picard without the entity. After determining that Picard is himself again, lacking the memories since he was taken over by the entity, the Enterprise continues on to Parliament.