The unidentified alien abducters were aliens that kidnapped Calvin in a single Sunday strip. After taking the boy to their saucer ship with a tractor beam, they isolated him in a life-support tube, studied him and used the research to create a robotic duplicate of Calvin. The doppelganger behaved mischievously; he stole cookies, broke a lamp with a shovel, and threw Calvin's math book in the garbage, all the time in plain sight of Calvin's mother.
The aliens appeared as medium-sized, frail-looking purple humanoids. Their head, which sits atop a long, slender neck, bears three eyes and an antenna. They have disturbing, seemingly permanent smiles on their faces.