On March 1, 1954, the U.S. tested the first H-bomb on the Bikini Atoll in the Marshall Islands. Having not warned Japan, a small fishing ship called the Diago Fukuryo Maru (Lucky Dragon No: 5) was sailing nearby and was hit by the fallout, resulting in the death of the ship's radioman. Most fish cought around Japan for the next few months was contaminated and could not be eaten.
Tomoyuki Tanaka, a producer at Toho Studios, developed an idea for a movie about a giant monster created by an H-bomb test that attacks Japan.
In the Kaiju Multiverse
Godzilla, a giant 50 meter monster created by the radioactivity from an H-bomb test, attacks Japan and completely destroys the country's capital city of Tokyo. The beast is finally killed by the Oxygen Destroyer, a device invented by Dr. Serizawa. The Doctor, however, sacrificed himself and died with the King of the Monsters in order to insure that his device would never be used again.
On Lagos Island, three man-made animals called Dorats are exposed to a nuclear blast and merge into one, giant monster: King Ghidorah.