Grendizer, Getter Robo G, Great Mazinger: Decisive Battle! The Great Sea Monster (1976)
Original title: グレンダイザー・ゲッターロボＧ・グレートマジンガー 決戦！大海獣
Grendizer, Getter Robo G, Great Mazinger: Decisive Battle! The Great Sea Monster is a 1976 Japanese-language action movie directed by Masayuki Akehi, with a running time of 31m. It stars Akira Kamiya, Hiroya Ishimaru and Keaton Yamada.
A giant sea monster known as Dragonsaurus has emerged from out of nowhere, terrorizing the depths of the oceans. Great Mazinger, Grendizer and the Getter Robo G team join forces to combat the new threat. But their task becomes more complicated when Dragonsaurus swallows up Boss Borot and makes its way towards Tokyo. Source: ANN