The Plot to "Godzilla vs. Gigan!"

 

A mysterious new amusement park called "Children's Land" is being erected in Tokyo, with a Godzilla Tower. A young manga cartoonist who is struggling to get ahead volunteers his services but he finds himself having doubts when he sees a young woman fleeing from the building, pursued by the same man he had talked to who was the head of Children's Land. In her haste, she dropped a tape. 
The cartoonist points her pursuers in the wrong direction; then picks up the tape and enters the building. The "chairman", a seventeen year old boy, tells him that the woman is an enemy of peace. The woman approaches him later that evening, accompanied by someone else: a long haired hippie sticking a gun in his back. He faints. He wakes up in his own apartment to find the woman and the hippie with him. It turns out that the hippie was in fact armed with an ear of corn!
They play the mysterious tape, alerting the owners and awakening the monsters on Monster Island. Godzilla tells Angorus to see what's going on. The three drive to a countryside house where they learn that the so-called "chairman" and his partner has been dead for a year. Suspicions aroused, the three drive back to Tokyo. 

Angorus appears in Sagami Harbor and is greeted with cannon shells and bright search lights in his face. Having had enough, Angorus returns to Monster Island. In the Godzilla Tower, the woman's brother is being held prisoner. The cartoonist sneaks back into the Godzilla Tower to ascertain his presence there. 
Having confirmed the woman's worst fear, Kotaka is then caught by the Children's Land policymaker. The man gives Kotaka a pack of cigarettes which, unknown to him, contain transmitters in the filters. The bad guys walk into the apartment and steal the tape, and as they threaten to kill the three of them, the cartoonist's sister walks in. She kicks all their butts with karate and they flee. The foursome go to the police with the information only to have their story poohpoohed.
The report arrives that Godzilla and Angorus have broken loose and are headed for Japan. The bad guys call Ghidrah from outer space and Gigan from Space-Hunter Nebula Universe M. The two space monsters head for Planet Earth. Gigan and Ghidrah proceed to destroy Tokyo; Ghidrah with its trademark lightning bolts spitting from its three heads, and Gigan with its belly saw. Their destruction spree is rudely interrupted when Godzilla and Angorus show up on the scene.

Meanwhile, the cartoonist, his sister, and the woman's brother find out the truth about Children's Land's policymakers; they are aliens from another planet. They are giant cockroaches who take on the form of human beings. At the film's end, Godzilla and Angorus' combined efforts succeed in beating back Ghidrah and Gigan; the Godzilla tower is knocked in to a pile of rubble; and the aliens are destroyed. Japan finds itself enjoying calm but not cleanup once again!