Hailing from 22,000 light years away from the Milky Way galaxy, the Messiah 13 aliens came from the Yomi system. The green skinned species covered most of their physical features. Their normal attire was a black and dark brown guise, including an odd, square shaped hood. This mostly covered their face, with two small slits for eyes. Their leader, the Hell Commander, wore a distinctive outfit from the rest of the rank. This included a Romanesque gold armor and he also had the lower part of his face exposed.
Just like Earth, the alien's home planet of Messiah 13 was the third from their system's sun. Unfortunately, as is the fate of all planets, their world began to die out. Confronted with this, the war-like race mobilized an attack plan to find a new home.
Discovering Earth and realizing its similarities to their home world, the advanced race enacted a plan of conquest. Utilizing their warship, the Daimakan, the invaders unleashed a fleet of Hell Fighters toward Earth. Masked as a comet scare, the aliens had the unintended luck of their own communication causing electric wave interference, hampering Earth's means of communication. Despite the aliens efforts to covertly strike, UFO sightings still poured in from the East Coast of America. In response, the UN Space Federation began to investigate, sending doctor Schmidt. Moving to their next phase of attack, the aliens deployed ground troops. These extraterrestrials eventually came into contact with Schmidt, killing him and presumably his party. The alien race also learned of the Goten, an advanced Earth battleship. Realizing its power, the race became focused on controlling it.
Being covert was a natural element of the Messiah 13 aliens' attack plan. Using their imitation abilities, one of their troops disguised themselves as Schmidt. This involved perfectly mimicking the doctor's appearance as well as voice. With his disguise, the alien soldier visited the home of professor Masato Takigawa, captain of the Goten. The arrival shocked the professor, who had been told moments before that his friend, Schmidt, had been killed. The false Schmidt assured him that he faked the report, but had been in great danger from an invading race. Attempting to leverage the friendship with Schmidt, the alien sought to convince Takigawa to launch the nearly finished Goten. This included handing over the battleship's plans in the process. The professor saw through the alien's disguise, though. The extraterrestrial overlooked the small detail that Schmidt was left handed, having held a lighter in his right hand. With his disguise compromised, the alien instead held the professor at gun point. With his weapon revealed, members of the UN Space Federation happened to enter Takigawa's house at the same time. Looking to abduct the professor, a light skirmish broke out. During the conflict, one of the Federation members managed to knock the gun out of the alien's hand. Another member secured the weapon, before shooting the alien in the shoulder. Wounded, the alien realized it would be quickly overwhelmed. As a result, it leaped through a ground level window, running off and killing itself in an explosion. Federation members came across the alien's mask, which allowed it to mimic Schmidt's appearance. They then came across the being's body, seeing for the first time the green skinned alien.
With their attack no longer a secret, the alien race opened a full scale war with humanity. Sending their Hell Fighters, the species decimated Earth's landmarks. Meanwhile humanity mobilized members of the Goten at its rocky construction site. The extraterrestrials wasted no time in ascending on the location. They proceeded to assault the island with Hell Fighters. After a day of aerial bombardments, the Messiah 13 aliens switched tactics. Sending in a squad of eight commandos, the invaders killed the three guards placed near the entrance. Quickly ascending on the fallen soldiers, the aliens mimicked their appearance. The invaders then entered the facility, unleashing a swift, brutal attack. Messiah 13 commandos slaughtered and mimicked as they advanced deeper into the base. Despite the loss of three of their ranks, they found their target: Takigawa. Still wanting to capture him, and secure the ship, the aliens once more held him at gun point. However, members of the Goten crew surprised the aliens from behind, killing the invaders with laser fire.
With the assault defused, the Goten finally launched. The Messiah 13 aliens responded with an all-out attack from their Hell Fighters. Despite superior numbers, though, Earth's warship easily destroyed the legion of alien fighters.
With Earth no longer under siege, the Goten descended into outer space. The craft was on a course for Venus, where they had detected communication from. On the way, the battleship came across the remains of Space Station Terra, which had been destroyed by the Daimakan. Among the wreckage, the Goten crew found the body of one of the station's staff, Tadashi Mikasa.
After bringing the body on board, a legion of Hell Fighters quickly appeared near the Goten. At the same time, the body of Mikasa came to life, revealing that the aliens had imitated him to board the Goten. Holding Jun Takigawa, the professor's daughter, at gun point, the alien moved toward the airlocks. As planned, a Hell Fighter docked, on-boarding the disguised alien and Jun. The craft then proceeded to retreat toward Venus with its captive. Earth's ship ignited its engines in pursuit, before receiving a message from the aliens. The Hell Commander, leader of the alien attack, proceeded to reveal himself to the Goten crew. He mocked their efforts, while revealing their intent and fate of their original home world. The alien leader then warned that their Earth ship would soon be drenched in blood.
The threat did little to deter the Goten, which arrived on Venus. After some minor survey work from the Land Rover, locating the Daimakan, the Goten crew mobilize a two stage attack. Their initial goal was to dismantle the barrier around the Messiah 13 warship. Their method included Star Fighters, to cause a cave-in above the Daimakan. The second stage involved ground troops to attack inside the Messiah 13 craft.
As the Star Fighters began their assault, the extraterrestrials responded by releasing Hell Fighters. Despite the Messiah 13 aliens' efforts, the Star Fighters were successful. With the barrier removed, ground troops ascended inside the Daimakan. The Goten crew encountered numerous Messiah 13 sentries aboard. Casualties piled up on both sides until only two of the original four Goten troops remained. Putting safety aside, the last two rushed the base to try and damage the ship's power. The attack was met with the death of one crew member, at the hands of Messiah 13 aliens hiding behind a wall bunker. With only one surviving member, named Koji Miyoshi, a legion of Messiah 13 aliens confronted him. Among their ranks was the Space Beastman, an alien monster, and the Hell Commander. The surviving Goten crew member attempted to attack, only for the Hell Commander to use his telekinetic cane. The weapon effortlessly threw Miyoshi to the ground several times. During the attacks, Miyoshi's gun and knife were knocked away. Disarmed, the surviving member and Jun were thrown in a jail cell together.
With his captives, the Hell Commander once more called Takigawa, asking him to surrender. The exchange was short, as Takigawa refused despite the risk to the hostages. The alien commander warned that there will be retaliation. Making good on his threat, the Space Beastman was sent to meet the captives. However, Jun, who observed the Hell Commander controlling the ship, deduced their hexagon control pattern. As a result, the captives freed themselves and killed the Space Beastman while escaping.
With his captives having fled back to the Goten, the Hell Commander mobilizes the Daimakan. Their first assault destroys the last Star Fighter before confronting Earth's battleship. The two giant crafts exchange fire and the Goten momentarily comes out ahead. However, the Messiah 13 aliens reveal the ship's Gravity Gun. With the secret weapon, the Goten is crippled, crashing to the surface below. Like the Daimakan, though, the Earth ship has a hidden weapon of its own. In a surprise move, the Goten launches its drill. Containing an Ether Bomb, the drill penetrates the Messiah 13 craft. Once inside, the weapon detonates. The Ether Bomb's effect is slow to start, first destroying the interior of the Daimakan. The weapon's attack spreads, though. It eventually destroys the entire planet of Venus, and with it the last remnants of the Messiah 13's attack force.