Shapeshifters are a special kind of aliens on the show, who can take on the appearance of different people. We’re first introduced to the idea of alien shapeshifters in the episode “The Convention”, from Everett Hubble, who’s figured out that his wife’s killer was a shapeshifter. In that same episode, we find out that Hubble shot an innocent man believing him to be the killer based on his appearance. Hubble also becomes convinced that Max is the shape-shifting killer because of his other alien abilities.
The existence of shapeshifters adds a lot of mystery to the show, because as long as there are shapeshifters around, it’s hard to be sure if anybody is who they seem to be. There are a few mysterious figures who have appeared in the show who were suspected to be shapeshifters, but were never confirmed, such as the man who shooed Max and Liz out of the area where they dug up the alien orb in the morning, in “Sexual Healing.”
In the episode “Foursquare”, Max falls into the same trap as Everett Hubble, and accuses Tess of being a shapeshifter after finding out that she has alien abilities. Finally they meet Nasedo, a true shapeshifter.
In season three, Max goes to Los Angeles to look for another shapeshifter, and finds Kal Langley, who’s given up shapeshifting in order to pursue a human life. Langley tells Max that the cost of shapeshifting is the pleasures of the senses, and that after years of staying in the same form, he’s regained a little bit of his sense of taste and is able to recognize the sourness of lemons. Max also finds out that shapeshifters from their ship are bound not to directly harm him and to obey his direct commands. He uses this to force Langley to shapeshift in an attempt to pilot an alien ship.