Archer, Sato and Reed are infected by a virus that mutates them into members of a long-extinct species, the Loque'eque.
While tracking the Xindi, Archer, Sato, Reed and T'Pol explore a planet and are exposed to a virus that rewrites their DNA. Archer, Sato and Reed begin mutating into the Loque'eque, an extinct species that created the virus to preserve itself, while T'Pol is only mildly affected.
The mutated crew lose their memories and speak an alien language, believing they must reach a lost city. Alien containment ships led by an official named Tret demand access to stop the virus. Phlox finds a cure, and Archer keeps the last vial as a remnant of the species. The episode was directed by LeVar Burton and written by Andre Bormanis.