Two Ultra Guard scientists, Doctors Segawa and Maeno create a new devastating weapon -- the anti-planet device they christen the R-1. Even in its infancy, the R-1 is said to have the power of 8,000 Hydrogen bombs. With plans to build an even more deadly R-2 device, it is decided a test of the weapons capabilities will be done on the supposedly uninhabited Gieron star, a minor planet in some faraway galaxy. The test proves successful, but a giant flying creature doesn't take too kindly to its home being destroyed. Making its way to Earth, the monster attacks cities and emits radioactive ash. Ultraseven engages the beast in battle.
“ギエロン星獣登場”
水爆の8,000倍の破壊力を持つ超兵器R1号を完成させた地球防衛軍。
地球防衛のための、最強の武器を手に入れた喜びに浮かれる警備隊員を尻目に、軍拡競争を憂い思い悩むダン。
超兵器R1号の実験のためにギエロン星が選ばれ、破壊力を試すことになった。
爆破実験は成功し惑星は完全に破壊されたが、実験時の放射能の影響で突然変異したギエロン星獣が地球に飛来する。
放射能を振り撒くギエロン星獣を倒すため、セブンが立ち向かうのだった。
