Deep in outer space, a toaster with several parts attached to it is seen flying through space. It soon drifts toward an unknown planet and ends up in the hands of a humanoid figure, exactly seven feet tall, clad in shoulder armor and wrist gauntlets, and with dreadlocks and four mandibles near it's mouth. The creature takes the toaster back to a room full of skulls, and with a large monitor in the center. The creature then hooks the toaster to the monitor, and on it plays a video of a big-headed boy and a robot dog, speaking of the planet Earth and of a possible peace agreement between Earth and space. Once the video is over, the creature leaves, grabs a metal mask from the wall, and heads to a spaceship and flies off.

Meanwhile, on Earth, the big headed boy, Jimmy Neutron, has snuck out of his home and met with his friends, Sheen Estevez, a spiky-haired boy wearing a shirt that says "Ultra Lord", and Carl Wheezer, a fat boy with red hair and coke bottle glasses. They spot what looks like a shooting star, and wish that they didn't have parents. Meanwhile, downtown, a patrol officer is at the bakery eating a donut and spots a beam of light which quickly disappears. He walks out, heads to where the light stopped at, and spots the alien ship. A door opens, and the creature from the alien planet steps out, unsheathes wristblades, and stabs the cop in the belly with them.

Meanwhile, Jimmy and the gang are enjoying themselves at the theme park known as "Retroland". After a half-hour, the boys leave the theme park and spot police cars heading into town. They head into town and find the remains of the police officer being loaded into a truck. They ask what happened to him, and the officers are unable to explain. The boys decide to do some investigating of their own. They arrive at the park and find the skinned remains of several cops hanging from tree branches. Sheen asks himself what Ultra Lord would do. Carl wretches, and uses an inhaler.

Just then, the boys hear a noise coming from the other side of town and quickly run there. They spot the creature battling cops and overwhelming them. Jimmy points a laser gun at it, and the creature responds by firing it's shoulder cannon in his direction. The beam misses, and the boys run. With no other option, Jimmy resolves to face the creature head on, but Carl and Sheen run away, leaving Jimmy to fight the creature alone. He gets into a robot suit and heads back into town to fight the creature, which, at this point, is collecting the heads of the cops that were battling it, most of which have the spines attached to them.

Jimmy arrives in the robot suit, and he and the creature battle, with the creature eventually destroying the robot, and Jimmy grabs a glowing knife from the wreckage, and the creature discards it's shoulder cannon and mask, and it and Jimmy continue their battle, with neither side able to best the other. Eventually, however, Jimmy stabs the knife in the creature's abdomen, killing it, then other creatures like it walk in and collect the remains of the other, and the leader of this group hands Jimmy a World War 2-era smatchet before joining the others. Once the creatures get onboard their ship and fly back into space, Jimmy returns home.