Godzilla vs the Sea Monster

 

 
This movie has the Big Green One fighting a gigantic lobster called "Ebirah" though you never hear the name in the English version. A storm in the Pacific results in a shipwreck. One of the men's brother takes it into his head to try and find him. He and two friends take a boat with a bank robber inside. Soon the four are sailing over the bright blue ocean in search of Rota's brother Yata. 

A storm blows up and they find themselves on an island. They also find out that they are not alone. There are also soldiers and a base on the island. A ship sails into the dock spraying a mysterious yellow liquid. It bears a cargo of captive natives from Infant Island whom the soldiers are using as slaves. A woman slave named Daiyo makes her escape when the huge lobster surfaces and terrorizes the islanders. 
She meets up with the four and enlists their aid. During a raid, the five are spotted and shot at. One is captured, Rota escapes in a weather balloon, and the remaining three return to the cave. They soon discover--you guessed it-- that they are not alone. Godzilla is asleep in a deep chasm in the cave. To rescue Nita, they decide to try to wake up Godzilla by spooling wire around Godzilla's dorsal spines and waiting for lightning to strike. Godzilla wakes up and is really very angry. He goes to fight Ebirah. 

At the film's end, Godzilla defeats Ebirah and goes after the soldiers. Mothra comes and rescues the captured natives. Godzilla wants to fight but Mothra has not come to tango. Godzilla leaps off the island just before it is completely engulfed in atomic flames.