During a birthday party for a young woman named Ruriko, a alien known as Braco walks out of the ocean, kills her dog and makes off with the girl. Anne, who is attending the party, alerts her comrades. Searching for her missing friend, they find the monsters footprints, but no sign of the creature. The Ultra Guard eventually find Ruriko unconscious on the beach with a strange parasite attached to her arm. They discover to their horror that these spores can only grow and multiply inside of female host bodies. The only cure is Radiation Alpha 73, a special element found on Saturn. With but hours to save her and Anne both, Ultraseven and the Ultra Guard must face an assault by dozens of aliens before Seven can race to the ringed planet to retrieve the element required to save the two women.
