Mognezun was a monster who had been feeding on the Yellow Gas (a real life world war II weapon) dumped by Japan's Army, giving the monster his poisonous gas powers. Mognezun used his gases to attack trespassers who neared his territory before MAT could react or arrive. After taking out one of MAT's members, the rest of MAT tried to take care of the situation, at first they seemed to get the advantage by placing a giant muzzle over Mognezun’s dangerous mouth, but Mognezun eventually pried it off and sprayed his gas at Goh, luckily Ultraman Jack appeared and the two fought. While Ultraman tried to fight the monster, Mognezun used his poisonous spikes to paralyze Jack and started poisoning Jack with his gas. MAT quickly returned with a new weapon, a gas machine that exhaled a white gassy substance that made Mognezun's poison flammable. Jack proceeded with firing the Specium Ray and igniting the gas. With Mognezun's gas effect nullified, Ultraman Jack was able to regain control of the fight and ultimately struck Mognezun in the head with his Meteor Kick, causing Mognezun to keel over and die.
映画のロケ隊が撮影現場で遭難した。イエローガスを吸いこんだことが原因だと聞き、岸田隊員は戦慄する。それは岸田の父が開発に関わった、旧日本軍の秘密兵器だったのである。極秘裏に廃棄されたはずのイエローガスのカプセルを怪獣モグネズンが食べ、ガスを吐き出していたのだ。岸田は単身、モグネズン退治に向かう。
