Hiroshima, summer of 1945. A young girl named Nobuko saves a starving kitten from crows. She wants to keep the kitten, but her parents dismiss the idea. Nobuko eventually convinces them to let her keep it. She and her brother Makoto name the cat Kuro. Kuro quickly brings joy and laughter to the family. As World War II takes its toll on Japan, it becomes harder for Nobuko and her family to care for Kuro. One August morning, Kuro begins acting strangely...

The first theatrical film of the anime series Kinnikuman. After winning the 21st Choujin Olympics, Kinnikuman plans to build his own theme park. As his friends and celebrities arrive, the sky grows dark and with a flash of lightning Kinnikuman's championship belt is gone. He then sees that it has been taken by Octopus Dragon III, who challenges him to take it back. Kinnikuman tries but fails and Mari is kidnapped. Octopus Dragon gives Kinnikuman a time limit to save her before she is fed to the monster Gigasaurus and then returns to his home world of Planet Metro (メトロ星, Metoro-sei). Despite his injuries, Kinnikuman is determined to go and reclaim his belt and Mari. Luckily, Terryman, Ramenman, Robin Mask, Warsman, Brocken Jr., and Rikishiman arrive to lend him a hand.