The Enterprise is caught in a mysterious void by a being, Nagilum, conducting an experiment to study humans.

Diana Muldaur (Dr. Katherine Pulaski)
Earl Boen (Nagilum)

Charles Douglas (Ensign Haskell)
Colm Meaney (Transporter Chief [O'Brien])

• This is the first episode where we see Worf's holodeck calisthenics program.
  
• Nagilum creates a reproduction of a Galaxy-class starship, which Riker identifies as NCC-1305-E, U.S.S. Yamato. The real U.S.S. Yamato will be seen later this season in "Contagion" (however, its registry will be NCC-71807).