Major Monkey hates living on Mars because it is cold, dry and there is no one for him to talk to. So he builds robots to keep him company. After discovering other evil masterminds trying to take over earth but being defeated by Ricky Ricotta's Mighty Robot, he comes up with a plan to get rid of him.
On Earth, after making the mistake of crushing Ricky's parents' minivan, Ricky and the Robot are searching for a way to pay for the damage when a spacemouse flies down on his ship and cries out for the Robot to save Mars. The Robot flies off to Mars, unaware that he is stepping into a trap.
The Robot lands on Mars, but finds no evildoers. Suddenly, a giant metal hand bursts out of a hillside and grabs him. The spacemouse, who is really Major Monkey in disguise, reveals himself and summons three huge Mecha-Monkeys, who follow him to conquer Earth.
Ricky is in the middle of a spelling test when Major Monkey and his thugs arrive. Worrying about what happened to his friend, Ricky asks the evil primate where the Robot has been. Major Monkey tells him the Robot "got into a tight squeeze", then seizes power over Squeakyville and demands the citizens' obedience.
Meanwhile, Ricky is worried about his friend, but a general from the Squeakyville Air and Space Association arrives and asks Ricky and his family to come. They hop into the general's rocket ship and fly over to Mars.
When they arrive, Ricky, his family, and the general discover the Robot held captive in the giant hand. Another hand rises up and grabs the rocket. Major Monkey's laboratory, which is actually a humongous Orangu-Tron, climbs up out of the ground. Ricky bravely sneaks into the Orangu-Tron and discovers the main power switch. He is spotted by a Robo-Chimp, but begins to pull at it with his breathing tube until the power goes out, freeing the Robot from the Orangu-Tron's grip. The Robo-Chimps close in on Ricky, but the Robot saves him. Thinking fast, Ricky presses a Self Destruct button, causing the Orangu-Tron to explode.
As soon as Ricky, his family, the general, and the Robot arrive on Earth, Major Monkey spots them and orders his Mecha-Monkeys to attack. The Robot fights back, and Major Monkey sics them on him again. After a scary battle, the Robot brings down the Mecha-Monkeys.
The Mecha-Monkeys retreat and fly back to Mars, and Major Monkey breaks down in tears, unable to rule the world anymore. Ricky tells him he must pay for his mistake before taking him to the Squeakyville jail and asking the general for a new minivan to take responsibility for the trouble he and his Robot had caused in the first place.