The starship Enterprise encounters a future version of itself that, due to a temporal phenomenon, had been sent back 117 years into the past. This Enterprise was originally identical to the "real" Enterprise, with the same crew, but had been sent back in time after entering an unstable subspace corridor. For decades it existed in the Expanse, never contacting Starfleet, waiting to catch up with the current Enterprise and warn its past self about the wormhole. The only crewmember to survive through the decades was Sub-commander T'Pol, who finds she has the privilege of meeting a less aged version of herself. All of the other inhabitants of the ship are descendants of the original crew (some of whom have interbred with friendly aliens they encountered along the way). For example, Lorian (David Andrews), the captain, is the son of Commander Tucker and T'Pol. As such the Enterprise has become a generational ship.
Archer and Lorian disagree over which method of propulsion Archer should use to transport his ship to a certain set of coordinates. Lorian believes that the wormhole must be avoided altogether, while Archer believes that his ship can pass through it without a repeat of the temporal disruption that sent her into the past because of modifications suggested by the elder T'Pol.
Meanwhile, the younger T'Pol encounters her older self and learns that in the alternate future caused by the wormhole she and Tucker are destined to be together. She also is told that the brain damage suffered from her Trellium-D addiction (cf. "Damage") is permanent. When Young T'Pol expresses confusion over what she should do regarding Tucker assuming the timeline is restored, her elder self says simply, "Follow your heart."
Later, in order to prevent Archer from using his warp drive, and thus using his preferred method of propulsion, Lorian steals a vital part of the warp drive (ironically echoing a similar act of theft Archer was forced to undertake with an alien vessel in "Damage"[original research?]). In the end, the crews are able to reconcile their differences and decide on going through the wormhole. Lorian assists by attacking some enemy ships guarding the wormhole. Archer's Enterprise successfully passes through towards its rendezvous with Degra, but Lorian's ship does not follow. A final exchange between Archer and T'Pol provides two possible explanations: either the ship was destroyed, or it ceased to exist because Enterprise made it through the wormhole without being sent into the past. Yet T'Pol is confused as to how the present day Enterprise remembers the Enterprise from the future if the ship never existed.
"Children of Time," the Star Trek: Deep Space Nine episode with a similar plot; the Defiant crew forms a colony two hundred years in the past, only for it to be erased in the present by the actions of one of the colonists.