Theodore was born in BabyCorp, and was going through a series of sorting tunnels that are required when a newborn rises. He is shown to be a little dull, as he gets baby powder on his face instead of his bottom, like the other babies. When he gets his booties, they are placed on his hands instead of his feet. Then Theodore spots a baby-bottle machine and realizes if he doesn't turn over onto his belly, the bottle will be in his bottom. He quickly turns over and the bottle lands in his mouth. Then he approaches a sorting tube, and the babies in front of him get tickled by a mechanic hand and get sent down the family tube. When it is his turn, he gets tickled on his tummy, and he doesn't feel a thing, since his tickle spot is his left foot.
Once he falls down the management tube, he is fitted into a suit, a tie, and a watch, and the mechanical hand places a briefcase in his lap. He is transported to his small office, where he opens his briefcase and finds a binder and four crayons.
After the introduction scene, a taxi with the BabyCorp logo arrives. A baby steps out and Tim is surprised. Soon he hears the doorbell ring and runs downstairs. His parents hold his baby brother to him, and he points them to go left, and they follow. Once they lay him on a sofa, he begins to cry so his mother gives him a bottle. He grabs it and slaps her hand, so she lets go of the bottle as he chugs it down.
In the next scene, it is shown he has meetings smack down in the middle of the house, and as he gets angry, he begins to cry. Janice and Ted kneel and ask him if he needed Senor Squeaky, and he grabs Senor Squeaky.
Scene after that, Theodore is throwing things at breakfast, and Tim holds up a plate as a shield.
In the middle of the night Janice and Ted are running around. Janice and Ted approach his room and Janice gives him fake play keys, he rattles them, begins to breath loudly and let out a horrible,, awful tantrum. They start running around much much more.
In the next morning, a butt lands on Tim's face, and it is Theodore's. Theodore is held up by Janice, and Tim states their both naked. Ted snaps a photo and Janice laughs.
At dinner, Janice is spoon feeding Theodore. Tim states he wears a suit, and carries a briefcase, and Theodore pushes his briefcase below the table. When Ted says Tim still carries Lamb-Lamb around, Theodore laughs and spits out a little of his baby food. When Ted says one day Tim will love Theodore, Tim goes up to Theodore and whispers, 'never.' Theodore pushes him away and he tumbles backward.
And to wrap up this history, after Tim hears Theodore is going to stay there forever if he doesn't find out how to stop the new puppy, the kids team up. They get their first lead by going to bring your child-to work day at the PuppyCo Building. It is revealed that the puppy never ages, like upper management babies, and Luis says he will hurt Janice and Ted if they don't step out of his way.
When Eugene becomes their babysitter, the kids escape to Las Vegas, where they are being chased by Eugene. Once there parents are trapped, Theodore and Tim defeat Luis and stop the rocket from launching.
Theodore gets promoted and his first letter arrives. It regards how Tim wants to give Theodore all his love. As he opens the package, thousands and thousands of beads fill the room. Theodore is so happy he jumps right out into his diaper, and when it is time for the tickling-hand, Theodore puts up his foot and gets sent down the family tube.