Yesterday's Enterprise (Star Trek: The Next Generation, Season 3, Episode 15)
Another standout time-travel episode. An Enterprise ship and crew from an alternate timeline come through a temporal rift. Coincidentally, this crew comes from a time when the Federation is at war with the Klingons. Captain Picard and his crew have to decide whether to send them back to their near-certain demise or risk disrupting their own timeline. Denise Crosby, whose character Tasha Yar was killed in Season 1, makes a comeback as a member of the alternate-timeline Enterprise crew.
Watch Star Trek: The Next Generation on CBS All-Access.