Jim wakes up to find an alien, Roy, who claims to be saving the human race, has inhabited his head, expanding it to unconventional proportions and alarming local scientists, feds, and extraterrestrials alike.
As the Feds' search for Jim intensifies, he works with Roy to recover the first of five pieces of a broken machine that could stop an invasion of new, parasitic aliens who want to destroy the human race.
Jim's quest to locate the second piece of the anti-invasion machine is hampered when evil adversary Gork steals it and cripples Roy.
Jim and Roy team up with Meredith as they venture to South America and battle natives to attain the third piece of the machine.
Roy sees that the Orb, the last piece of the machine, is already in the hands of Gork and the Feds. After Jim and Roy retrieve it, they are shocked to realize a spy among their own ranks helped the enemy capture both Roy and take back the Orb.
With the spy now on his side, Jim strengthens his forces in preparation to rescue Roy from Fed possession. A race ensues between Jim's team to save Roy and capture the Orb before the evil space aliens can use it to destroy the human race.
In a final confrontation with the brainwashed Feds and Evil aliens, Jim's team dominates and stops the alien invasion.
While playing basketball with Roy, Jim sees a baby that's about to be hit by a car so he rushes to save it -- resulting in press coverage for his heroic deed that's seen by a sinister agent. So when Jim appears at open mic night at a local bar, the agent arrives with contract in hand and turns Jim into an overnight celebrity. Jim gets a big head (so to speak), so Roy peruses the contract and discovers Jim has actually sold his soul to the devil. Madelyn and Roy make a deal with the prince of darkness. Can they beat an American Gladiators-style contest in Hell?
During a lunch with the human anomalies group, Chin reveals that he's been plagued by nightmares, so Jim suggests that he goes to China to confront his demons. Shortly after arriving, Chin vanishes on Jim and Roy, so they go into a bar looking for him, but instead discover his father. Meanwhile, Chin has been hypnotized by the evil Mr. Chang, his former captor, who makes Chin fight in a freakshow as The Spider. When they catch a commercial with Chin for an upcoming battle to the death, it's up to Jim, Roy and Chin's father (the original Spider) to save him.
A pair of Siamese twins woo Raquel and propose to her. She's swept off of her feet... unaware that they're secretly evil.
Jim awakens to find that his already gigantic head has grown even larger -- so large, in fact, that it's wedged in the four corners of his bedroom ceiling. A visit from Dr. Axel results in the draining of a green ooze from Jim's brain and a prescription for a stress free vacation. Jim, Roy, Ivan, Ray and Dr. Axel head out on a fishing trip to find the legendary giant blue fish. Shortly after their departure on the water, the gas tank gets punctured and the boat explodes, leaving them stranded on a remote island. Soon Axel and Ivan are kidnapped by an evil old man who breeds the blue fish... and they discover that the fish live off of human flesh.
An alien bearing a great resemblance to Roy breeched “the security of Technology Incorporated Laboratories and absconded with a container of dangerous experimental energy” (or anti-matter). Pasquale Mendoza alerts Jim and Roy to these events when he comes knocking, assuming that it was Roy who was responsible for the theft… but soon Jim and Roy spot another man with an unusually large cranium – which contains Roy’s baby brother, Mark. Mark is supposed to deliver the anti-matter to a race of nasty, green, humanistic aliens who need the substance for fuel, but a botched transaction on the moon leaves Roy as a hostage to the nasty aliens.
