Peter and Lois visit Lois' sister Carole who is alone and pregnant since her husband walked out on her. While at the house, Carole goes into labor and Peter is forced to deliver. Overwhelmed by the experience, Peter and Lois decide to have another baby. A shocked and appalled Stewie however, makes it his mission to stop any conception whatsoever.

Through varied attempts and schemes, Stewie is able to avoid any colloidal collisions between Peter and Lois, but when they lock him out of the room, he's forced to go into emergency mode. Stewie shrinks himself down to microscopic size and seeks out his new sibling (sperm-brother) inside Peter's body. Instead of killing his unborn brother, they hit it off - bonding over the desire to destroy Lois. Stewie leaves Peter's body with the new hope that his brother to-be will soon join him in his attempts at world domination.