The Beta Renner Cloud is a living nebula, inhabited by disenbodied intelligences which use it as a physical body. It is able to travel at warp speed, and its conciousnesses can get separated from it and inhabit other types of matter, including spaceship systems and humanoid minds. One of these non-corporeal intelligences was accidentally separated from the cloud when the starship Enterprise passed through the system - an experience that left the non-corporeal being terrified.
The entity entered the minds of several crewmembers before finally settling in Captain Picard's, and ordering the ship back to Beta Renner, where it attempted to merge with Picard's consciousness, converting him into an energy lifeform in order for him to become a permanent inhabitant of the cloud as well. The two intelligences proved incompatible, however, and the captain managed to return to the Enterprise and use the physical pattern stored in the transporter's memory system to regain his body.