The stories of Blaster Master and Metafight are vastly different. In Blaster Master, Earth is threatened by an alien race who has made its home deep underground. The hero, Jason, finds a path to this underground lair after attempting to retrieve his escape pet frog Fred. Fred comes into contact with radioactive waste, which causes him to mutate and grow to several times his size. In order to defeat the mutants, Jason dons a combat suit and pilots a mobile tank called S.O.P.H.I.A. the 3rd, which can be fitted with many attachments to help it progress through the vast underground network.