This 1964 film is a compilation of the first two episodes of the 1957 Japanese serial Supa Jaiantsu. It's also known as Starman, Attack of the Flying Saucers, and simply Atomic Rulers.
The people of the Emerald Planet grow distressed with Earth's continued flirtations with atomic testing. They fear that radiation from Earth will travel to their galaxy and destroy their planet. So they send Starman to earth to destroy any atomic devices he happens to stumble across.
Starman is a flying superhero, complete with suit and cape and disguises himself as Japanese business man (with a deep American accent). No sooner does he arrive in Japan does he discover that criminals from the country of Mergolia are smuggling an atomic device into Japan. Starman (in his business suit disguise) fights the criminals, but the atomic device falls into the hands of a young orphan boy who just happens to be playing nearby. He escapes, but his friend is captured by the Mergolians.
Starman follows the boy to retrieve the device, but Mergolian thugs try to recover the device (one of the thugs looks exactly like Richard Nixon).
Starman has to protect the atomic device, protect the orphan, save the friend and stop the Mergolians plan of world conquest while wearing leotards that are clearly two sizes too small. He even takes time out to flirt with a nun.
This is light but cheesy fun. The special effects are crude but occasionally interesting. There's a lot of backwards filming in the fight scenes but it's done to good effect, especially during a sword fight scene.
What's interesting is this yet another example of the evils of atomic weapons portrayed in Japanese popular culture. Part of this could simply be an attempt to cash in on the success of Gojira with its atomic subtext, but clearly this is another cautionary tale warning mankind of the nuclear bogeyman, playing on the fear of the time that science may be going a step too far. We humans may not be wise enough to recognize the dangers of releasing the nuclear genie from the bottle, but clearly the enlightened beings on the Emerald Planet do, and they must save us from our own ignorance.