Koutarou and Clan are working their way back home from their interstellar adventure, but they have to make a pit stop about ten years off from their intended destination. As luck would have it, they encounter a young girl right after they land. Worried that she might have seen their spaceship in action, Koutarou decides to dig a little deeper. Things take an unexpected and heart-wrenching turn, however, when he realizes he has more in common with this girl than he initially thought.

Just starting high school, Koutarou is moving out on his own to take some of the burden off of his widower father. Lucky for him, he's found a room at Corona House. It has a great landlord, it's not too far from school, and best of all, it's dirt cheap. It really is perfect... except for the strange girls that keep appearing to try and take it over! But even as room 106 turns into a battlefield, Koutarou isn't willing to give up his apartment without a good fight. The invasion begins!

After getting back from Forthorthe, everyone was hoping they could spend their days in peace for a change. But as if the politics of the official Forthorthian invasion weren’t enough, something very strange and very unsettling is starting to happen around Koutarou and the girls. One mysterious disappearance becomes two, and there’s no clear cause or end in sight. Can they figure out what’s going on in time, or will Koutarou end up just like he was in the beginning—alone?

Valentine's Day is right around the corner, and the girls of room 106 are scrambling in preparation for it. It's not all fun and games, however, because not everyone seems to be able to admit their true feelings. Ruth receives a strange message from her parents back home in Forthorthe, leaving her with an important decision to make about her future. And on top of everything else, she seems to be getting closer and closer to figuring out what happened to Koutarou while he was away.

Koutarou and the girls prepare to chase Ralgwin back to Forthorthe, but one critical mission remains before they can leave Earth behind again—they must oversee the welcome ceremony for the next wave of intergalactic transfer students. Little do they know that someone is watching their every move from the shadows, someone with a keen interest in both Koutarou’s sword and Nalfa... Indeed, the Gray Knight is getting dangerously close the group and the truth about what happened back in the ruins. Will their rematch with him be their final showdown?

It’s been two full years since Koutarou first started at Harukaze High as a freshman and a battle for his apartment turned his life upside down. A lot has changed since then. His former rivals are now his closest friends, and they’ve even added a few new faces to the group. Revisiting things like the inter-club obstacle marathon during the sports festival as third-years is going to be a wild ride, compounded even further by the danger lurking just around the corner...

Deciding they need a bit of a break, Koutarou takes the girls out for a day of beetle hunting. An unexpected reunion, however, leads Kiriha to realize there’s more than just beetles in the mountains outside of Kisshouharukaze City. It seems the radical faction has been working in the shadows all this time, and they’ve already established a foothold on the surface. But a factory? Magicians? Automated weapons? It sounds like they’re preparing for something big, but this is only the beginning.