There was once an ogre (oni) mother who gave birth to a human looking child. Instead of an ogre's horn, this child was born with a sword in hand called the "Onikirimaru" or Ogre Slayer. He has no name and is only referred to by the name of his sword. Though he appears human, he is pure ogre. Believing that he will become human once all of his kind have vanished from the world of humans, he sets out on his destiny to slay all of his kind. Even now, in the mother world of Japan, he is still out there slaying ogres.

The story takes place between the first and second season, where Rimuru confronts the swirling conspiracies in the Coleus kingdom.

Kim is living a peaceful life with his father as a farmer on their planet until one day when a runaway spaceship is detected moving at a high speed towards his father's farm. To rescue his father Kim jumps on board the ship and manages to land it safely before it crashes. On board the ship he finds a severely Injured lone survivor whom with his dying words begs Kim to take something of his to the Galactic Fleet. He then transfers something that was attached to his arm onto Kim's arm. The thing was a Lens and its transferring should have been impossible and having it has turned Kim into a Lensman. Other than giving Kim unknown powers it also contains vital information for the victory of the Galactic Fleet over the evil Boskone Empire. Kim must now using the spaceship Britannia bring the Lense to the Galactic Fleet. But this is not an easy task when Lord Helmet of the Boskone Empire is willing to use everything in his power to stop him.

After battle in first season battleship Amanohara was serious damaged and need to repair. It land on island where a Gogyou festival just begin. Umakai Shintaro and his five girls student who know as Raimu Unit begin enjoy a peace time in that island. But the enemy still follow them around. How Umakai Shintaro and his five girls student fight against the enemy. Still his five girls student begin try to draw his attention into them.