A group of scientists and reporters have went on an expedition to Oilis Island, and find a rare Miloganda plant, but were almost attacked by a huge green plantlike monster (known as Greenmons), which they shoot to death. But one month after their return to Japan (with the Miloganda plant at a greenhouse lab), the very same Greenmons murders all of the expedition members, except one: Photographer Setsuko Hamaguchi, who the Science Patrol suspect will be the monster's next target and place her under protective custody. They also find that radioactive energy (even their Superguns and Arashi's Spider Shot gun) can make the monster stronger . . . and grow! What is the connection between the Miloganda plant and Greenmons? Will Ultraman stop the green terror from causing any more destruction?