Battle In Outer Space primarily serves as a followup and a sequel to the events and ideas presented in Ishiro Honda's earlier alien invasion film, The Mysterians. As such, several years have passed since the Mysterian invasion rallied Earth's forces under one banner to fend off the alien menace. In that time, Earth has prepared for future attacks from the deep reaches of space via satellites and space stations orbiting the planet, monitoring the stars for activity. However, with all their preparations, some incoming ships quickly destroy the orbiting satellite, and before anyone knows what is going on, Earth is once again under attack! But from whom?
Around the globe, numerous devastating disasters occur, including a railroad bridge levitated off the ground causing a train wreck in Japan; an ocean liner lifted out of the Panama Canal by a waterspout, destroying it; and severe flooding in Venice, Italy. A UN-connected international meeting is called at the Space Research Center in Japan. Major Ichiro Katsumiya, Professor Adachi and Dr. Richardson open the conference and describe the disasters, adding that the survivors suffered from extreme frostbite. Dr. Richardson theorizes that some unknown force lowered the temperatures of the objects so as to lower the Earth's gravitational pull, thus making the objects easier to lift, regardless of their size and weight. Katsumiya determines that such an action could only be accomplished by a force beyond the stars.
Dr. Ahmed, an Iranian delegate at the meeting, reacts as though suffering from a severe headache and slips away. Ahmed walks outside to a courtyard in a daze. Etsuko Shiraishi sees him and watches in horror as he is enveloped in a red light coming from the sky. Astronaut Iwamura comes in and Etsuko tells him what happened but Ahmed is nowhere to be seen. Back at the conference, it is believed that aliens might be behind the disasters and it is suggested that the Earth be prepared militarily. Dr. Ahmed appears and tries to sabotage the heat ray experiments held at the meeting, apparently under some sort of mind control. In the ensuing scuffle, he briefly takes Etsuko hostage and warns that the Earth will soon become a colony not of Mysterians, but from the planet Natal. Just then, a Natal saucer appears near the center and vaporizes him, but forensics find a tiny radio transmitter that was put in him, receiving signals from an alien base located on the moon.
The UN decides to launch two rocket ships, called SPIPs, to the Moon on a reconnaissance mission. En route, both ships are attacked by remotely controlled meteors called 'space torpedoes'. Iwamura, the navigator of SPIP-1, is also under mind control by the aliens. He is caught trying to disable the rocket's weapons and is tied up. Both SPIPs avoid the meteors and are given a warning by the Natal not to land on the moon, but it is ignored. Once the rockets land on the moon, the two teams look for the alien base in lunar rovers. Meanwhile, Iwamura has untied himself and blown up SPIP-1. They find a cave on foot and locate the Natal base in a deep crater. Etsuko is temporarily captured by the Natal but is rescued by Katsumiya. A beam weapon battle erupts as the teams attack the base. The Natal base is destroyed, freeing Iwamura from the aliens' mind control. Feeling guilty, Iwamura stays behind to give covering fire, allowing the SPIP-2 to escape.
Back on Earth, the world prepares for a final conflict against the Natal. Scout Ships (based on the North American X-15 experimental rocket plane) and Atomic Heat Cannons are built to counter the invasion fleet. Eventually, the Natal saucers and their mothership approach Earth. Squadrons of Scout Ships (converted into Space Fighters) are sent up into space and engage in a massive dogfight with the saucers. The Natal mothership launches Space Torpedoes that hit New York and San Francisco. The mothership descends upon Tokyo and lays the metropolis to waste with its anti-gravity ray. The remaining saucers and mothership advance on the Space Research Center, but the Atomic Heat Cannons finally destroy the mothership and the Earth is saved - though, with major casualties and damages.
Battle In Outer Space is interesting, in that it feels largely like a remake of its predecessor, The Mysterians, albeit far more finely tuned. One part invasion/disaster film, and one part epic space opera, this is a film far ahead of its time. Its epic space dogfights predate anything seen in Star Wars by well over 20 years, along with some truly stunning scenes of destruction during the attack sequences. Unfortunately, despite being a fine movie, it does not have anywhere near the iconic power of The Mysterians, as the Nartal have a cool design, but a forgettable one - and there are no kaiju-sized robots on the march to give it a big memorable moment. Overall, it's a largely forgettable experience that has remained largely forgotten against Toho's cavalcade of monsters, but still fun enough and interesting in that it provides a glimpse into what it might mean for man to travel in outer space several years before the real life moon landing was accomplished.