Wesley's science project accidentally releases microscopic Nanites into the ship's computer core, where they are causing system failures and threaten Dr. Stubbs' lifelong experiment.

Patrick Stewart (Capt. Jean-Luc Picard)
Jonathan Frakes (Cmdr. William Riker)
LeVar Burton (Lt. Cmdr. Geordi La Forge)
Michael Dorn (Lieutenant Worf)
Gates McFadden (Dr. Beverly Crusher)
Marina Sirtis (Lt. Cmdr. Deanna Troi)
Brent Spiner (Lt. Commander Data)
Wil Wheaton (Acting Ensign Wesley Crusher)

Whoopi Goldberg (Guinan)

Ken Jenkins (Dr. Paul Stubbs)

Mary McCusker (Nurse)
Randal Patrick (Crewman #1)

• Dr. Crusher returns from Starfleet Medical and Dr. Pulaski is gone for good. Gene Roddenberry said of Diana Muldaur "The chemistry we had hoped for did not develop....Our producers were happy with her and hope she will be available for future guest roles." There were also the rumors that she did not get along with the rest of the cast and crew. Rather than look for another doctor, they asked Gates McFadden if she was interested.
  
• No explanation is given for Dr. Crusher's return to the Enterprise.
  
• Geordi La Forge has been promoted to Lt. Commander; Worf has been promoted to full Lieutenant (from Lieutenant junior grade).
  
• The uniforms now have a small, raised collar and no zipper down the front. For the men, the top is now separate from the bottom as opposed to the one-piece spandex jumpsuit; the female officers still wear a one-piece uniform. The pips (rank insignia) are now on the collar.
  
• Beginning in Season 3, Gene Roddenberry hands over day-to-day operations to Rick Berman. Michael Pillar takes over the writing staff.
  
• The opening sequence has also changed. Instead of zooming out from Earth to the Enterprise, the camera starts outside of the galaxy and zooms to the Enterprise.