Jun, lppei, Yuriko and friends get lost in a thick fog driving back from a party. In need of shelter, they find an abandoned mansion in the middle of the swamplands. This reminds Jun of a story of a Baron who was obsessed with spiders and went mad when his daughter went missing in the swamp after a poisonous tarantula bit her. According to legend, she and her father turned into Giant Spiders.

Disaster strikes when the first manned spacecraft surveying Saturn crashes off Japan's coast. As Jun and Yuriko fly over the area, they run low on fuel and return to find a strange, floating blob. It expands, destroying their car and injuring Ippei. Hovering over Tokyo, it absorbs all kinds of energy. Professor Naramaru's suggests the UN launches a satellite to lure Balloonga into space.