Big Baby is one of the villains in the stop-motion show Action League Now!
One day at a local carnival, The Mayor is hosting a freak show. Quarky, Thundergirl, and Stinky Diver are there and watching the show. The Mayor presents a gigantic infant known as Big Baby. Thundergirl thinks the baby is cute but Stinky thinks the baby is ugly. The Mayor pokes Big Baby and tells him to dance for the people. Big Baby grabs the pencil and tosses The Mayor into a nearby trash can. Big Baby then breaks out of his cage before scaring the people away and grabbing Quarky.
At a Mobile Command Post, the Action League is discussing how to stop Big Baby. Stinky is not above destroying him but Bill the Lab Guy protests. He says Big Baby is a rare creature to be studied and that they should use reason not violence. The Chief and Thundergirl tell Bill that Big Baby grabbed his daughter, Quarky. Upon hearing this, Bill snaps and wants to destroy Big Baby.
Meanwhile, Big Baby goes on a rampage towards a nearby house where the family is having lunch. Big Baby tears the roof off the house and scares the family. Before Big Baby can attack them, Stinky and The Flesh arrive in the Action Mobile. Stinky is about to shoot Big Baby but The Flesh tells Stinky he he got Big Baby mad. Back at the Mobile Command Post, Bill grieves over Quarky being taken by Big Baby. The Chief tries to comfort Bill by saying he's got his best people on the matter. Suddenly, Big Baby tosses the Action Mobile at the Mobile Command Post.
Moments later, Big Baby climbs up a coat rack while still holding Quarky. Thundergirl flies to the rescue. She asks Big Baby if he wants his pacifier. Still angry, Big Baby punches Thundergirl and she lands in a cactus. Quarky starts begging for her father to save her. Bill wants to save Quarky but he doesn't want to hurt Big Baby. He tries to think of a solution. The Flesh and Meltman are having a hard time thinking because a stench is in the air. That suddenly gives Bill an idea. The Action League figures out why Big Baby is so cranky: he needs a diaper change. Stinky Diver climbs up the coat rack and uses his spear gun to remove Big Baby's dirty diaper. Justice opens the trash can - where The Mayor is still inside - to dispose of the dirty diaper. The Flesh then rides a skateboard and rams the coat rack. This causes Big Baby to fall. Meltman and The Flesh catch Big Baby in a new, clean diaper. As Quarky falls, Thundergirl tries to catch her but misses. Quarky still manages to survive the fall.
When Big Baby becomes calm, The Chief is impressed at what a difference a clean diaper can make. The Flesh (who is now wearing a diaper) agrees with him. Quarky is happy to see her father again and Bill is relieved that she's all right. Stinky feeds Big Baby by using a bottle of formula. Thundergirl tells Stinky she thought he hated babies. Stinky tells Thundergirl he thought so, too, but now he finds Big Baby to be cute. Big Baby spits the formula onto Stinky and everyone laughs.