The USS Enterprise is en route to the neutral world of 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, Worf is zapped with strange energy as the ship passes the cloud, causing him to become violent. Dr. Crusher sedates Worf and brings him to Sick Bay, but she is also infused with the same energy as she examines his body. Dr. Crusher begins to act oddly to those around her and goes to the bridge, asking questions about the ship's navigational functions. When she questions Data at one of the science stations, the energy sparks between her and the computer console, leaving Dr. Crusher slightly confused as to why she's on the bridge. The ship suddenly starts to have malfunctions and Captain Picard orders Assistant Engineer Singh to investigate the cause. Singh is later found dead near a computer link, and Picard orders a murder investigation, considering the alien delegates to be prime candidates. Meanwhile, Counselor Troi evaluates both Worf and Dr. Crusher, finding neither recall their strange behavior.
The ship suddenly drops out of warp, and as Picard investigates the readouts, 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 themselves back into the cloud, and he shoots energy at the bridge crew when they try to stop him. They are unable to prevent Picard from beaming off the ship. However, Counselor Troi senses Picard's essence nearby, and Picard himself 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 onto Parliament.