In 1988, Earth was invaded.
Hailing from the Third Planet of Yomi, the Messiah 13 Aliens began a sneak attack against humanity. Initially mistaken for a comet scare, the extraterrestrials caused massive electric wave interference on Earth. This disrupted global communication, hindering efforts to investigate the cause. Word, however, got out of UFO sightings on the East Coast of America. As a result, the United Nations Space Federation sent doctor Schmidt to investigate. The trip was Schmidt's last, though, as the aliens killed the UN representative and take over his appearance.
During the excursion, the Messiah 13 learn of the unfinished battleship Goten. Correctly analyzing the immense power of the Earth ship, the extraterrestrials became focused on gaining control of it. Their first effort involved exploiting the friendship between Schmidt and professor Masato Takigawa, captain of the Goten.
Sending their infiltrator, the alien met professor Takigawa at his home, surprising the man as he had been told of Schmidt's demise. The impostor, however, explained that the report was fake, done to escape from an invading alien race. Although growing skeptical, Takigawa listened as the impostor tried to convince his "friend" that they needed to complete the Goten. That the ship must launch against the aliens. Takigawa was now in the position to decline the mobilization of the battleship twice in the span of a day, having turned down earlier a request from colleague Koji Miyoshi. However, where as his earlier refusal was based around a fear of the sheer power of what the Goten was capable of, his next refusal was on different grounds. As the imposter asked for the battleship's plans, Takigawa noted that he could not hand them over, admitting he had seen through the disguise. As it turns out, the aliens overlooked the small detail of Schmidt being left handed. This fact, combined with the mannerisms and magnitude of the request, gave their impostor away.
With their original plan foiled, the invader pulled out a weapon and held Takigawa at gun point. Just then, other members of the UN Space Federation entered the room. They returned to the area after seeing what appeared to be the deceased Schmidt heading to the Takigawa residence. In short order, a fight broke out, as the alien was both disarmed and wounded. Fleeing, the invader killed itself in an explosion outside. With the seriousness of the threat apparent, Takigawa and the UN Space Federation focused their efforts on launching the Goten.
No longer a secret, the Messiah 13 launched an open war on Earth. All over the globe, Hell Fighters appeared and quickly decimated cities and landmarks. The assaults were not enough to stop key members of the Goten crew from arriving at the battleship's rocky construction site, though.
Switching their plan from capturing the ship to simply destroying it, aliens descended where the Goten was kept. As work was rushed underground to get the warship ready, Hell Fighters bombed the island from above. With their attacks unable to destroy the ship, though, a different tactic was employed. Sending a squad of commandos, the extraterrestrials struck the base on foot just before the Goten's launch. The siege was almost successful, although the aliens switched plans once more to kidnapping Takigawa so they could control the ship. Consequently, members of the Goten team snuck up on the Messiah 13 beings, killing them and saving Takigawa.
The last obstacles out of the way, the Goten was finally launched. Using its front laser cannon, the ship blew a hole from the underground base to the surface. Powering its diamond fiber infused drill, the craft emerged above ground. With their greatest fear realized, the invaders ordered their Hell Fighters to lay waste to the Earth ship. Despite their efforts, though, the Goten's armor was too strong. Unfazed by the cannon fire, the giant battleship launched aerial depth charges in retaliation. Each charge exploded in a deadly show of fire power that obliterated nearby Hell Fighters. After a mere seven charges, the alien attack force was no more. Humanity was sparred to live another day.
The Goten's mission wasn't finished, though. Wanting to take the battle to the aliens, the giant warship ventured into outer space. Having previously detected communication originating from Venus, the Earth craft set a course for the distant planet, venturing out on the near 261 million kilometer journey. During its trip, the Goten discovered the remains of Space Station Terra, a giant orbital structure that had been destroyed by the Messiah 13 Aliens and their Daimakan warship. Although the crew debated if they should explore the wreckage, eventually crew member Reikai Muroi convinced the captain to let him depart the ship to investigate. While rummaging through the remains, Muroi discovered the body of a former staff to the station, Tadashi Mikasa. After bringing the corpse back to the Goten, a group of Hell Fighters suddenly emerged near the Earth battleship. The Messiah 13 beings had set a trap, as the body of Mikasa sprang to life, revealing it was nothing more than an alien in disguise. Grabbing the captain's daughter, Jun Takigawa, the invader was able to escape on a Hell Fighter, the Goten crew too afraid to attack for fear of Jun's safety.
As the alien vessel sped off with its captive, the Goten didn't wait long to ignite its engines in pursuit. Chasing the craft and heading toward Venus, the Earth battleship received a video message from the aliens. Introducing himself as the Hell Commander, leader of the Messiah 13, the extraterrestrial briefly lamented the destruction of their own home world. This destruction created the need to find a new home: Earth. Any sympathy their plight might have had, though, was quickly washed away as the Hell Commander then foretold that the Goten would be drenched in the crew's blood if they were to venture further.
Adamant to end the threat to Earth, the Goten ignored the warning and arrived on Venus. Deploying a Land Rover from the Earth battleship, the vehicle began a reconnaissance trip to understand what they were up against. The smaller vehicle was successful at both locating the Daimakan and also discovering a barrier that protected it.
The Goten crew formulated and executed an attack plan to rescue Jun and damage the alien forces. The first strike involved sending out a legion of Star Fighters, launched from the Revolver Cannon on the Goten. Their goal was to cause a cave-in where the Daimakan was hiding, destroying the barrier in the process. The second strike involved the Land Rover, which would deploy ground troops that would venture inside the alien fortress. As Hell Fighters emerged and an aerial dog fight commenced, heavy causalities mounted on both sides. Eventually both strikes were successful. With Jun rescued, the Land Rover bid a hasty retreat to the Goten. Meanwhile the last airborne Star Fighter retreated as well.
Unfortunately, the Daimakan was ready for battle. With a single blast from its Space Beam Cannon, the final Star Fighter was destroyed. Realizing their trip had been leading up to this moment, the Goten flew into battle. The two, giant warships exchanged fire, at first appearing evenly matched. After a fierce exchange, though, the Goten was able to come out ahead. The Earth ship first damaged the Daimakan with its Revolver Beam before destroying the alien's Vice Cannons and multi-dimensional stabilizer using the diamond fiber drill. Looking to end the fight, the Earth ship unleashed an incredible attack. Concentrating firepower from its laser cannons, Active Radar Missile and Revolver Cannons, the alien ship was greatly damaged.
Smoking and in bad shape, the Daimakan opened a port on the deck, revealing the ship's Gravity Gun. The immensely powerful weapon turned the tides of battle in an instant, striking the Goten and causing massive damage. As cannons and other offensive features were disabled, the giant Earth ship crashed.
Seemingly beaten, the captain of the Goten secretly ventured to the drill of the ship. He revealed the frightening power of the battleship, as the drill launched at the Daimakan. Its intent was to unleash its Ether Bomb. The aliens did their best to stop the attack, but their Gravity Gun was unable to halt the diamond fiber drill. The sharp object burrowed into the alien warship before detonating. The severity of the attack slowly built, first destroying the alien ship from the inside out. The damage spread, though, from the ship to the planet itself. As the Goten left the planet, Venus begins to erupt in smaller explosions. Eventually, the attacks mounted into a mammoth explosion, destroying the entire planet and ending the threat of the Messiah 13 aliens.