ENT Season 3 Episode 8 Twilight

Plot

While rescuing Sub-Commander T'Pol from a spatial anomaly, Captain Archer is infected by subspace parasites in his cerebral cortex, resulting in anterograde amnesia. His condition prevents him from forming new long-term memories. This allows him to remember everything prior to the accident, but any new memories fade within a few hours. It soon becomes clear that Archer is not fit for duty, and he is subsequently relieved of his command. T'Pol is granted a field commission to Captain, but the mission fails and Earth, alongside every other human colony, is destroyed by the Xindi weapon. The few surviving humans form a convoy, led by the Enterprise, which travels to the planet Ceti Alpha V.
Twelve years pass and Archer, still plagued by memory loss, lives with T'Pol in a house on the colony. She has given up her career to care for him. They are visited by Doctor Phlox, who eventually engineers a cure. He also discovers that when the subspace radiation treatments kill one of the parasite clusters in Archer's brain, it also vanishes from every other previous medical scan – as if the parasite had never existed. Therefore, since Archer will never have been infected, he would have remained Captain and possibly prevented the chain of events that led to Earth's destruction.
Unfortunately, the ship, now captained by Captain Tucker, is observed and attacked by Xindi vessels before the treatments can be completed. The Enterprise is outnumbered and heavily damaged. Phlox, T'Pol, and Archer race to create a subspace implosion and as their procedure nears completion, Phlox, T'Pol and the entire bridge crew are killed. Archer himself is fatally wounded but manages to complete the procedure. The ship is destroyed, but their plan works and the subspace parasites are also destroyed by the implosion, and wiped out through time. The timeline is reset; Archer is in sickbay recovering from a physical injury but will never develop the amnesia he originally suffered.