Starfleet history records that in 2344, the Enterprise-C responded to a distress signal from a Klingon outpost on Narendra III that was under attack from Romulans. The battle went badly for the Enterprise-C and the ship was destroyed with all hands, but its sacrifice was regarded with the highest honour by the Klingons and greatly contributed to the formation of the alliance between the Klingon Empire and the Federation.
In the episode Yesterday's Enterprise, the Enterprise-C suddenly finds itself transported through time 22 years into the future as it is engaged in battle with the Romulans. Because of this spontaneous temporal shift, history is radically altered: the Klingons and the Federation did not form a treaty and instead went to war, a war that the Federation would inevitably lose.
When history was altered, only Guinan of the Enterprise-D is aware of the change and attempts to convince Captain Picard that the Enterprise-C has to go back to its own time, even though doing so means certain death for its crew. Picard informs Captain Garrett of the situation and that the Klingons are winning the war, and that the addition of one more ship isn't going to turn the tide. Garrett agrees with Picard and Guinan, realising what is at stake, and so she resumes command of her vessel and is ready to take them back through the temporal rift. At that moment, however, a Klingon Bird-of-Prey appears and attacks. An explosion occurs on the bridge and Captain Garrett is killed when a piece of shrapnel hits her in the head.
After Garrett's death, her helmsman Richard Castillo assumes command of the Enterprise-C. With the aid of the Enterprise-D's tactical officer Tasha Yar, Castillo succeeds in taking his ship back through the rift, restoring the original timeline.