While on a routine mission, the U.S.S. Enterprise discovers a bizarre rift in space, which reveals an unidentified vessel. Both ships are now on an alternate time line where the Federation is in the midst of a bloody war with the Klingons.
The crew identifies the mysterious vessel as their ships' immediate predecessor, the U.S.S. Enterprise NCC-1701-C, believed to have been destroyed in battle over 20 years ago. Their own crew now includes Tasha Yar, the ex-security chief who was killed in the line of duty. Meeting with the captain of that vessel, Picard learns that it was responding to a distress signal on a Klingon outpost when it was attacked by four Romulan Warbirds. But in the middle of the assault, the ship broke through the space rift, thus saving it from the destruction recorded in the history books. Although Picard and his crew are baffled by the strange appearance of the U.S.S. Enterprise-C, Guinan's alien instincts lead her to believe that the Federation's current war with the Klingons is the result of the U.S.S. Enterprise-C's presence in their future. Certain that the ship is now existing in the wrong time line, she pleads with Picard to send the U.S.S. Enterprise-C back through the rift in order to change the course of events, which has resulted in the loss of 40 billion lives during the Klingon conflict.
Picard confers with Data, who asserts that if the U.S.S. Enterprise-C was destroyed while aiding the Klingon outpost, it would have been viewed as an honorable act, which means the current war with the Klingons may have been avoided. Faced with this knowledge, Picard decides to send the U.S.S. Enterprise-C back through the time rift, to face its tragic destiny.
Meanwhile, Tasha learns from Guinan that, in the correct time line, she died a meaningless death. In response, Tasha requests that she be allowed to serve on the doomed U.S.S. Enterprise-C crew so that she may die in the heroic battle with the Romulans. Picard reluctantly agrees.
As the U.S.S. Enterprise-C makes its way back through the time rift, three Klingon warships launch an attack on it. To assure that the U.S.S. Enterprise-C returns to its rightful place in time, Picard positions the U.S.S. Enterprise-D between the vessel and the Klingons, sustaining heavy losses in the process. Just as the U.S.S. Enterprise-D's engine core seems on the verge of exploding, Data announces that the rift has closed, and, unaware of the preceding events, the U.S.S. Enterprise returns to its original time line, heading for a cultural exchange program with a Klingon delegation.