Yesterday's Enterprise

A temporal rift opens and the Enterprise-C, a ship lost decades earlier, emerges. Instantly history changes: the Federation is losing a long war with the Klingons, the Enterprise-D is a warship, and Tasha Yar is alive at tactical. Only Guinan senses that this is wrong, and she urges Picard to send the older ship back, though that means certain death for its crew. Captain Rachel Garrett explains her ship was answering a Klingon distress call when Romulans attacked. She agrees to return, but is killed in a Klingon raid, and Lt Richard Castillo takes command. Learning from Guinan that in the other timeline she died a pointless death, Tasha transfers to the Enterprise-C. As Klingon battle cruisers attack, the Enterprise-D holds them off while the older ship escapes, and Riker is among the dead. The Enterprise-C passes through the rift and history is restored. Guinan, sensing something, asks La Forge to tell her about Tasha.