The Enterprise-D docks at Starbase 133, where Dr. Crusher greets her elderly friend and mentor, Dr. Dalen Quaice. After taking him to his quarters discussing the loss of old friends, Dr. Crusher visits her son Wesley Crusher in Engineering. Wesley attempts to create a static warp bubble but the experiment appears to fail. As the Enterprise leaves Starbase, Dr. Crusher finds that Dr. Quaice seems to have gone missing, with no record of him coming aboard the ship. As she performs a medical test on transporter chief O'Brien, she realizes she is the only one in Sick Bay; further investigation and discussion with the crew shows that she has always worked alone in Sick Bay.
Dr. Crusher continues to try to track down the disappearing people, and finds more and more crewmembers that she remembers being completely unknown to the crew or the computer. At one point, a vortex appears near Dr. Crusher and attempts to pull her in, but she is able to hold on to a fixture until it dissipates; the ship shows no record of the vortex' appearance when she investigates. Eventually, no one but Captain Picard and herself remain on the ship, Picard believing that the situation is normal. Dr. Crusher orders the computer to play Picard's heartbeat over the ship's speakers so that she knows he is still there, but shortly thereafter even Picard disappears.
Alone on the ship, Dr. Crusher realizes that The Traveler, who had previously helped the Enterprise out of a similar situation, may be able to help, and sets course for his homeworld. En route, the Enterprise comes to a standstill, the computer no longer recognizing the name of the Traveler's homeworld. Dr. Crusher quickly realizes that the universe itself appears to be shrinking, and soon, the computer only recognizes the Enterprise as the extent of the known bounds of the universe. As the universe continues to shrink, Dr. Crusher recognizes the shape of the universe to be the same as the warp bubble experiment Wesley was working on and surmises that she is trapped inside a warp bubble that is rapidly collapsing. She races ahead of the moving event horizon to Engineering, where another vortex has opened. She jumps through it to find herself back in Engineering surrounded by Wesley, Picard, and the Traveler. After confirming that everything is back to normal, including her guest Dr. Quaice, she learns that after the crew discovered her trapped in the warp bubble, they called upon the Traveler to help Wesley open a portal out of the bubble to save her.
An interesting side note is that this episode was heavily influenced by several chapters at the end of the classic science fiction novel The Demolished Man, by Alfred Bester