On a mission to investigate an abandoned Xindi vessel on a jungle planet, Archer, Reed and Hoshi succumb to a virus that mutates them into a primal life form. Meanwhile, Enterprise is threatened by alien vessels that aim to eradicate the virus.

Roger Cross (Commander Tret)
Daniel Dae Kim (Corporal Chang)
Troy Mittleider (Private Palmer)

Philip Boyd (Comm Officer)
Brian J. Williams (Alien Decon Agent)
Craig Baxley Jr. (Decon Agent #1)
Keith Schindol (Decon Agent #2)
Jimmy Ortega (Decon Agent #3)

• Beginning in this episode, the series' title in the opening credits changes from Enterprise to Star Trek: Enterprise.
  
• The mutagenic virus was created by the extinct Loque'eque species. It causes the infected host's DNA to mutate into Loque'eque DNA. The Loque'eque engineered the virus when their species lost the ability to reproduce and was going extinct.
  
• Phlox creates an anti-virus. They give the serum to Tret, the captain of the containment vessel.
  
• This episode was dedicated in memory of Jerry Flick, first assistant director.