Telepathic aliens leave crew members comatose and haunted by memories.
The Enterprise welcomes a group of Ullians, telepathic historians who retrieve and record memories. Soon after, members of the crew begin to fall into comas. Counselor Troi, Dr. Crusher and Riker each experience disturbing memory-based nightmares, and suspicion falls on one of the Ullian visitors.
The story is a thriller with an uncomfortable theme, the violation of a person's mind. It is considered one of the more disturbing episodes of the series, with a focus on the effects of mental intrusion. Picard has to decide how to handle an apparent crime committed by a guest. It was a well-crafted story that used the characters' memories and fears to create tension.