Robbie is Rob and Chris' infant son. Chris had presumably given birth to him before the events of the film. With Robbie (known as "the Little Master" by the appliances) as a new member of the family, Toaster and the gang fear that the Master and Chris will pay more attention to him, but they eventually get used to the Little Master during the "I See a New You" sequence.
One night, the Hearing Aid, who was left in a junk drawer from his previous owner, gets out and, assuming everyone is asleep, talks to someone in space, making Toaster and the others suspicious. The next night, Robbie suddenly awakens, gets out of his crib, and follows the Hearing Aid. The appliances soon find the Little Master going up the stairs. Lampy tries to get him down, but is dragged up the stairs and falls down after his plug slips out. A big beam of light appears in the kitchen just as the appliances enter the room. The appliances chase after the Hearing Aid, but then the Little Master enters the kitchen, gets abducted by the beam and disappears.
The appliances restrain the Hearing Aid and discover that Robbie was sent to Mars, according to the Hearing Aid's readout on the Calculator. The appliances contact Wittgenstein, their old college buddy, and he gives them advice on how to get to Mars. After the appliances get the equipment they need, the Hearing Aid volunteers to go with them, and they travel to Mars to find the Little Master.
When the appliances arrive on Mars, they find Robbie trapped inside a bubble. They later learn that the Supreme Commander and his rejected appliances intend to destroy Earth because their human owners had thrown them out when they no longer worked. While Toaster tries to talk them out of it, the Little Master manages to push a hand out of his bubble, which touches the Supreme Commander, and the refrigerator inexplicably turns pink. He smiles at the child, but then returns to his original color and pushes the baby aside.
In an attempt to save the Little Master, Toaster runs against the giant refrigerator in an election for the title of Supreme Commander. In the aftermath, Toaster wins the election and becomes the new Supreme Commander, while the ex-Supreme Commander is revealed to be the Hearing Aid's long-lost twin brother. When asked by Toaster why he changed his mind about blowing up Earth, he says that "the touch of a small boy's hand" reminded him that not all humans are bad.
After the Little Master is rescued, the appliances return to Earth just in time as the Baby Monitor that Ratso, the family's pet rat, had been restraining all night, finally alerts Rob and Chris. They rush into the baby's room and discover Robbie walking all by himself, much to their joy, and then assume he learned it during the night.
One day while Rob and Chris are filming Robbie, the baby finds Tinselina (an ACAM that the appliances befriended on Mars and who chose to go back with them) in a garbage can, and she gets fixed up just in time for Christmas. Robbie then attempts to talk and says, "Toaster!" his first word. He then brings in Toaster, Kirby, Lampy, Radio, and Blanky, to which Chris turns to Rob and remarks, "Like father, like son." It is a happy ending as Tinselina spends her first time on top of a Christmas tree, and Robbie and the appliances have a merry Christmas with their Master and his family.