Dr. Crusher discovers crew members are vanishing one by one, but no one left on board misses them — or even knows who they are. She eventually discovers that she is trapped in a warp bubble generated by one of Wesley's experiments gone awry.

Eric Menyuk (The Traveler)
Bill Erwin (Cmdr. Dalen Quaice, M.D.)
Colm Meaney (Lt. Miles O'Brien)

• The Enterprise is docked at Starbase 133 for crew rotation. It is carrying 1014 people, including Dr. Quaice.
  
• Dr. Crusher interned under Dr. Quaice on Delos IV. Dr. Quaice has been stationed at Starbase 133 for six years.
  
• Dr. Crusher pages Dr. Selar and asks the computer to locate her; she goes missing soon after Dr. Quaice (see "The Schizoid Man").
  
• This was the 79th episode of Star Trek: The Next Generation. The original Star Trek series' episode count was 79.
  
• The Traveler was introduced in the episode "Where No One Has Gone Before". Wesley was experimenting with Kosinski's equations from the same episode.