When Theia's Blue Knight detects a strange anomaly near Earth, it looks like Koutarou and the others have a new kind of invader on their hands. Another princess from Forthorthe has come to sabotage Theia's trial, but no one seems to be able to find her! The threat lingers as they get involved with the drama club and agree to help put on a very special play for the upcoming cultural festival. And just how long can Yurika keep a you-know-what in room 106 a secret from Ruth?

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?

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?

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.

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.

Inept with a sword, unable to ride a horse, and apathetic toward the empire he serves, Regis Aurick is a hopeless soldier who spends his days buried in books. Banished to the borderlands, he encounters a stirring young woman with red hair and ruby eyes. She is Fourth Princess Marie Quatre Argentina de Belgaria, known simply to Regis as Altina. “…Do you want to become my tactician or not?” Having sought him for his rumored strategic prowess, she aspires to reform an empire led by self-seeking nobles and driven to pointless wars, the legendary blade of an emperor, the Grand Tonnerre Quatre, at her hip. Continuously underestimated by her regiment, including her would-be tactician, the princess resolves to prove her worth beyond a shadow of a doubt—no matter the risk. Here is a tale of war, love, and politics, woven by the bookworm and the sword princess.