FBI agent Topolsky returns to warn Liz that a covert alien hunter inside the Bureau is pursuing Max and his friends -- and that he will stop at nothing to get what he wants. Meanwhile, Isabel befriends Tess, a new female student who immediately arouses Max and Michael's suspicions.

Michael and Maria flee from Roswell after helping Laurie DuPree escape from the police, and Max learns that a deadly parasite was released into Earth's atmosphere when their ship crashed 50 years ago.