Gynaecologist Doctor Alex Hesse has a nightmare about urinating infants in a library. In the real world, he and fellow gynaecologist Doctor Larry Arbogast have invented a fertility drug, "Expectane", that is supposed to reduce the chances of a woman's body rejecting an embryo and thus avert a miscarriage. Unfortunately, they are not allowed to test the drug on women and move forward in their research. In response, Hesse breaks into the laboratory and locks himself in. The head of the review board, Noah Banes, informs Arbogast that while the food and drug administration has denied their team the right of human experimentation, the team has managed to receive a donation by Doctor Diana Reddin from the ovum cryogenics department. When Hesse questions the chances of a woman taking an unapproved drug during pregnancy, Arbogast reveals that there is no need to identify the gender of the experiment and convinces Hesse to impregnate himself, using an ovum codenamed "Junior".
That night, Hesse has another nightmare in which his potential offspring has his own face pasted onto it. That day, he complains to Arbogast that his nipples are hurting him. Later, the normally aloof Hesse inexplicably lightens up and chats incessantly about walks, massages, and naps. Reddin tells Hesse that being a woman is not as great as it sounds, citing the menstrul cycles which don't stop until menopause, while Arbogast warns Hesse of a disease that is turning men into strudels. Meanwhile, Arbogast's pregnant ex-wife wants him to be the doctor delivering her baby. Hesse begins to wonder what it would be like to be a father and watches some television commercials to have himself a good sobbing. He later begins overeating, with Arbogast's ex-wife noting his practice of "mixing cuisines". It is revealed that the "Junior" ovum is actually Reddin's own body, making her the mother of Hesse's child. Banes wants to take credit for the experiment despite having no role in it. Arbogast disguises Hesse as a woman and hides him in a retreat for expecting mothers outside the city, passing off his masculine appearance as past anabolic steroid use. Eventually, Hesse goes into labor (as does Arbogast's ex-wife) and gives birth via caeserean section to a baby girl naming her Junior. Arbogast delivers his ex-wife's child and the two reconcile to raise their new son as their own. Before the credits, it shows the two families on a beach on vacation celebrating the birthdays of Junior and Jake (Arbogast and his ex-wife's son), we discover that Reddin is pregnant and when the credits are rolling, they are all playing in the sea.