Jeff appears to return from space in "Holy Shit, Jeff's Back!", but he acts strange, causing Hayley some doubts, although his new attitude soon wins over Stan. In reality, he is an alien named Zebleer from a species that "collects" and he has come to sample more earth items, his race having dissected Jeff in order to find out more about him. Hayley and Roger board the alien mothership and find that the real Jeff has been killed previously and when they are captured, they are due to be next. Having bonded with Stan over froyo, Zebleer confesses what is going on and escorts Stan to the ship to save Hayley, noting that the froyo makes mankind worth protecting. When Hayley rejects his direct offer to replace Jeff, he instead elects to have Jeff's brain put in his body, trading his life. As they return to earth, Roger informs them that they will have their memories erased when they depart the spaceship, leaving them unaware that Jeff is no longer entirely human.
When Jeff and Hayley decide to have a baby between episodes "Bahama Mama" and "Roger's Baby", they find they cannot get pregnant due to Jeff's alien physiology. Roger becomes a surrogate to "re-birth" Jeff into a fully-human adult, although a hormonal imbalance nearly causes Roger to flee with the embryonic Jeff and rename him "Tristen." After the situation is resolved, Jeff is reborn as fully human, although he decides he is too young to have a child.