It's 1999. Humans have a base on the moon and send rockets there on a regular basis. And all the monsters on earth have been confined to "Monsterland", an artificial environment created specifically for them. It's monitored by an underground base. Among the island's inhabitants are Godzilla, Rodan, the larval Mothra, Manda (from ATRAGON), Angurias and Gorosaurus (from KING KONG ESCAPES). When they try to leave there's a preventive measure that discourages them from taking off. It's like a Kaiju wildlife habitat. We see Rodan dive into the water and capture a dolphin for food. The other Kaiju don't seem to fight with one another. But then, aliens, in the guise of human women, come and take over Monsterland, putting radio devices in both the workers-and the monsters-to control them. Rodan soon appears in Moscow, leveling the city. Mothra is in China and Godzilla blasts New York City with his radioactive breath. The people who work on the moonbase are called back to Earth, to investigate what has happened. They try to prevent more destruction. The aliens want to create a civilization for themselves and don't want humans to bother them. Eventually, mankind gets in control of the monsters-and they attack the underground alien base. This is when the invaders bring in their big weapon, the space monster Ghidorah, to battle them...
This is definitely one of the more entertaining Godzilla movies, so if you're in need a "giant monster fix" this will sate you. Recommended.