The Enterprise-D encounters a rift in space-time while on a routine mission. As they monitor the anomaly, the heavily damaged USS Enterprise-C, a ship believed destroyed more than two decades earlier, emerges. As the Enterprise-C clears the field, the Enterprise-D undergoes a sudden and radical change. The Enterprise is now a warship, and the Federation is fighting a losing war with the Klingon Empire; Worf disappears. Instead, Tasha Yar, who had been killed years earlier in the line of duty ("Skin of Evil"), still mans the tactical station. Guinan is the only member of the crew to sense something has changed but is unable to immediately determine the cause; she voices her concerns to Captain Picard, who heeds her warning but cannot act without more evidence.
Captain Rachel Garrett of the Enterprise-C and her crew learn they have travelled into the future, where the Federation is losing a long war with the Klingon Empire. Garrett explains that they were responding to a distress call from the Klingon outpost on Narendra III, but were attacked by Romulan Warbirds; the next thing they knew they were in the future. Though Riker insists on making the Enterprise-C combat-worthy to assist in the Federation's war, Picard recalls Guinan's advice and instead orders his crew to help assist in repairs of the Enterprise-C to allow it to return to the past, believing that its disappearance may have led to an alternate timeline that culminated in the present Klingon war. During the repair efforts, Yar becomes close to the Enterprise-C's helmsman, Richard Castillo. Guinan reveals to Yar that she should not be here; in the proper timeline, she died a pointless death and the two never met. Yar requests a transfer to the Enterprise-C based on Guinan's advice, which Picard agrees to.
As the Enterprise-C's repairs are completed, a Klingon scout ship discovers their location, and three battlecruisers are sent to attack the ships. With the anomaly becoming unstable, Picard orders the Enterprise-C to return, and uses the Enterprise-D to protect it. Both Enterprises suffer major crew losses under the Klingon barrage, including the death of Captain Garrett and most of her bridge crew on the Enterprise-C. With the Enterprise-D nearly destroyed, Lieutenant Castillo manages to guide the Enterprise-C fully through the anomaly, causing reality to shift back to the way it was before. The event appears to the normal Enterprise-D crew as a brief appearance and disappearance of a ship resembling the Enterprise-C. Guinan, still subtly aware of the reality shifts, asks La Forge to tell her more about Yar.