Residents are horrified when reports of gargantuan skeletal remains and giant footprints lead them to believe that an abominable snowman may be lurking in the nearby mountains. When the press is alerted, Yuriko is visited by her friend, Ayako, who relates a desperate story. While helping her fiance, an entomologist named Koji, collect insect specimens, they discovered a large species of Morpho butterfly that was thought to be native only to the Amazon. When the strange insects expelled their poisonous pollen on Koji, he became ill and fled from Ayako as though he were insane. When she finally caught up with him, she was horrified to find that he was mutating into giant wildman. Aided by Dr Ichinotani, the investigative trio pursued the trail of the lost Koji. When sighted, Koji has grown to a monstrous height and appeared to have lost his human identity. The frightened masses viewed him as a monster and the military was called upon to stop him. Ayako desperately reaches out to Koji, trying to revive any memories that he may still have of her. Dr. Ichinotani employs the X beam, a concentrated atomic heat ray designed to counter the effect of the Morpho poison, and before Koji can harm Ayako, he is driven back into the forest. Following his trail, Jun and the others find that he has been restored to normal size and is happy to be reunited with Ayako.
The Giant is one of the two giant human kaiju to appear in the Ultra series, the other being Giant Member Fuji in Ultraman.