A series of mysterious disappearances leads to the discovery of a huge radioactive monster living in the depths of the ocean.
This, the first of the Japanese giant monster movies, is reputed to be even better in its original Japanese edition, which I have yet to see. The American version adds footage of Raymond Burr talking to Asian actors, only interacting with the characters from the original movie if they could find an actor or actress who resembled them from the back, or if he talked to them on the telephone. There is a certain amount of dubbing, but it is kept to a minimum; the scenes without dubbing are handled by having another actor explain to Burr (whose Japanese is a little rusty) what the other characters are saying. Despite the fact that these tactics to Americanize a movie rarely work, this is one of the more successful examples along these lines; the movie is not ruined by them.
As for the Japanese footage, it's pretty impressive stuff; the model work that went into the making of these movies was much more effective in black and white, and one really gets the feeling that the entire city of Tokyo is being destroyed by the monster, not just a street here or there. It is also one of the few monster movies of this sort to emphasize the human suffering as well as the physical destruction caused by the monster; since Godzilla is at least partially a metaphor for the A-Bomb, this is unsurprising, as Japan remains the only country to have actually been attacked with such a weapon. The scene where a childrens' chorus sings a hymn to the dead and dying is quite moving.
Many other Godzilla movies have been made; none of them have had this impact.