As Enterprise heads toward its rendezvous with Degra and a fateful meeting with the Xindi Council, the crew faces a dangerous trek. To reach their destination, they must enter a subspace corridor guarded by the Kovaalans, a species that isn't very tolerant of trespassers. As Archer attempts to devise a game plan, the crew detects another ship. Strangely enough, it's an NX-class Starfleet vessel ... and as it approaches, it becomes apparent that its designation is Enterprise NX-01!
The ship's captain, Lorian, hails Enterprise and tells them to alter their heading. He then comes aboard Archer's Enterprise and tells the captain that the current mission will not be successful. If Archer takes his ship into the subspace corridor, it will be thrown back in time 117 years. This, Lorian says, is what happened to his Enterprise. His ship has been in the Delphic Expanse for over a century and he's here to prevent it from happening again. Lorian goes on to explain that his crew attempted to destroy the first Xindi probe, but were unsuccessful. Now, he wants to make sure Archer reaches his rendezvous with Degra and prevent the second attack from happening. Lorian's plan is to modify Enterprise's injector assembly using alien technology to enable the ship to reach high warp speeds for brief intervals. That way, Enterprise will be able to avoid the subspace corridor altogether.
Archer wants to believe Lorian, but he can't help but be skeptical. Lorian and his first officer, Karyn Archer, have Phlox confirm that they are indeed descendants from the original crew — Karyn is Archer's great-granddaughter and Lorian is the son of T'Pol and Trip. Archer decides to proceed with Lorian's plan.
As the two crews work together to modify the current Enterprise, everyone learns a little bit about their future selves. Trip enjoys bonding with his half-Vulcan son, but is unsettled to learn that he died when the boy was a teenager. Hoshi learns that she had two children, while Mayweather finds out that he ended up with MACO Corporal McKenzie. Reed is somewhat disappointed to learn that he remained a bachelor. On the other end of the spectrum, many of the crew are descendants of Phlox, who had nine children. Archer, meanwhile, enjoys some quality time with his great-granddaughter, who reveals that Archer's wife was an Ikaaran woman named Esilia, whom he rescued from an anomaly field. Karyn also has another surprise for Archer — she leads him to a senior officer's quarters on her ship, where someone wants to say hello. Inside the quarters, Archer is shocked to find an elderly T'Pol. The elderly T'Pol has apparently been influenced by her time living with humans — she embraces Archer and remarks that it's good to see him. She also hands him a PADD and asks him to give it to her younger self.
The younger T'Pol examines the PADD and explains that her older self doesn't think Lorian's plan will work. She's discovered a discrepancy in her son's calculations, and is afraid there's a chance that Enterprise could be destroyed if it exceeds a certain warp speed. The elderly T'Pol believes Enterprise should try the subspace corridor — it may be possible to reconfigure the impulse manifolds, which should prevent the corridor from destabilizing and history from repeating itself.
Archer tells Lorian that he's going to proceed with the elderly T'Pol's plan. Lorian, however, is unconvinced it will work and is afraid that Archer is just dooming Enterprise and humanity all over again. Lorian secretly forms an alternate plan — he will forcibly take the injectors from Archer's ship so that he can reach the rendezvous point and meet with Degra himself. Lorian takes the injectors, but Archer's on to him and the two Enterprises engage in battle. Lorian is ready to do whatever it takes to accomplish his mission, but Karyn Archer makes him see reason — the people on the other Enterprise are family. She won't allow Lorian to kill them. Finally, Lorian stands down.
Back on the current Enterprise, Archer confronts Lorian about his actions. The captain of the future Enterprise finally reveals the reasons behind his intense devotion to this mission. He devoted his entire life to preventing the Xindi probe from launching, but in the end, his only option was to send Enterprise on a collision course with the probe. His emotions took over — he couldn't sentence his crew to death. The Xindi probe launched all over again and seven million lives were lost. Archer sympathizes with Lorian, but tells him that all that matters now is what they do next. Archer wants to take Enterprise into the subspace corridor and complete his mission. It will be a lot easier if the two crews work together.
As both Enterprise crews get to work, T'Pol finally has an encounter with her elderly self. The older T'Pol tells her that the emotions she accessed with her trellium experimentation will never fully go away. She must learn to embrace them and live with them. The elderly T'Pol also hints that there's someone onboard who can help the younger T'Pol with these emotions — Trip. The younger T'Pol is a bit uncomfortable with this idea, and says that she doesn't know what she wants. The elderly T'Pol, however, assures her that she will someday.
Once the modifications to the current Enterprise are complete, the two ships head for the subspace corridor, with Lorian's vessel shadowing Archer's. Working together, the two ships manage to fight off the Kovaalan vessels, but as the Kovaalans re-group, Lorian tells Archer to head for the rendezvous. His ship will distract the Kovaalans, engaging them in battle, and he will follow Archer as soon as he can. Archer agrees and successfully navigates his way through the corridor. Once he's on the other side, however, he doesn't hear from Lorian's ship again. T'Pol speculates that the other Enterprise did not survive the battle, but Archer isn't so sure. Perhaps the fact that Enterprise successfully made it through the corridor means that history has corrected itself, and Lorian's Enterprise never existed. But that poses the question: why would they remember them?
Before they can dwell on this for too long, Enterprise is hailed — it's Degra. The time to face the Xindi Council has finally arrived.