'Yesterday’s Enterprise' (Story Code 163) by Ira Steven Behr, Richard Manning, Hans Beimler and Ronald D. Moore From a Story by Trent Christopher Ganino and Eric A. Stillwell

The U.S.S. Enterprise is on a routine mission when the crew encounters a mysterious rift in space. As an unidentified ship emerges from the phenomenon the crew of the Enterprise immediately changes: their uniforms alter, the bridge becomes dimly-lit, and deceased security officer Tasha Yar reappears at her station. However, Captain Picard and his crew are unaware of the transformation - unknown to them they are now in an alternate timeline in which the Federation is engaged in a violent war with the Klingon Empire. The mysterious vessel is identified as the U.S.S. Enterprise NCC-1701-C, believed to have been destroyed in battle over twenty years ago; when Picard meets the captain he learns that, after responding to a distress signal from a Klingon outpost, the ship was attacked by Romulan Warbirds, but managed to escape destruction by breaking through the rift. Meanwhile, the enigmatic Guinan begins to suspect that all is not as it should be; realising that the appearance of the Enterprise-C has somehow changed history, resulting in the loss of forty-billion lives during the Klingon conflict, she attempts to persuade Picard to send the anomalous ship back through the rift. Data determines that if the Enterprise-C had been destroyed while aiding the Klingon outpost, its crew's sacrifice would have been seen as an honourable act, and the war would have been avoided. Picard makes the tough decision to send the Enterprise-C back through the rift - but this time it will have an extra crewmember: Tasha Yar...

Patrick Stewart (Captain Jean-Luc Picard), Jonathan Frakes (Commander William Thomas Riker), LeVar Burton (Chief Engineer Lieutenant Commander Geordi La Forge), Michael Dorn (Security Chief Lieutenant Worf), Gates McFadden (Doctor Beverly Crusher), Marina Sirtis (Councelor Deanna Troi), Brent Spiner (Lieutenant Commander Data), Wil Wheaton (Ensign Wesley Crusher), Whoopi Goldberg (Guinan), Denise Crosby ( Security Chief Lieutenant Natasha ‘Tasha’ Yar ), Christopher McDonald (Lieutenant Richard Castillo), Tricia O'Neil (Captain Rachel Garrett)

Directed by David Carson
Co-Produced by Hans Beimler and Richard Manning
Produced by Ira Steven Behr
Executive Producers Rick Berman and Gene Roddenberry

TX:
19th February, 1990

Notes:
*Featuring Captain Jean-Luc Picard, Commander William Thomas Riker, Chief Engineer Lieutenant Geordi La Forge, Security Chief Lieutenant Worf, Doctor Beverly Crusher, Councelor Deanna Troi, Lieutenant Commander Data, Ensign Wesley Crusher, Guinan and Security Chief Lieutenant Natasha ‘Tasha’ Yar