Gaira, a humanoid sea beast spawned from the discarded cells of Frankenstein's monster, attacks the shores of Tokyo. While the Japanese military prepares to take action, Gaira's Gargantua brother, Sanda, descends from the mountains to defend his kin. A battle between good and evil ensues, leaving brothers divided and a city in ruins.
| Actor | Role |
|---|---|
 Kenji SaharaDr. Yuzo Mamiya | Dr. Yuzo Mamiya |
 Kumi MizunoAkemi Togawa | Akemi Togawa |
 Russ TamblynDr. Paul Stewart | Dr. Paul Stewart |
 Jun TazakiGeneral Hashimoto | General Hashimoto |
 Kipp HamiltonSinger | Singer |
 Yoshibumi TajimaOfficer Hirai | Officer Hirai |
 Nobuo NakamuraProfessor Kita | Professor Kita |
 Hisaya ItoChief Izumida | Chief Izumida |
 Nadao KirinoLt. Col. Kazama | Lt. Col. Kazama |
 Ren YamamotoSaburo Kameda | Saburo Kameda |
 Kōzō NomuraGeneral's Aide | General's Aide |
 Shōichi HiroseSoldier | Soldier |
 Tadashi OkabeDefiant Reporter | Defiant Reporter |
 Hideo ShibuyaSkeptical Reporter | Skeptical Reporter |
 Ikio SawamuraFisherman #1 | Fisherman #1 |
| SSeishirō KunoFisherman #2 | Fisherman #2 |
| SSomeshō MatsumotoOfficial | Official |
 Gorō MutsumiDr. Paul Stewart (voice) | Dr. Paul Stewart (voice) |
 Haruo NakajimaGaira | Gaira |
| HHiroshi SekitaSanda | Sanda |
| YYasuhiro KomiyaAdolescent Sanda | Adolescent Sanda |
| KKasei KinoshitaAdolescent Sanda (voice) | Adolescent Sanda (voice) |
 Yū Fujiki(uncredited) | (uncredited) |