With the help of his friend Kenji Matsudaira, Kōtarō Satomi moves in to room 106 at Corona House, attracted by the low, ¥5000 a month rent. The following day, while working at an archaeology site, he falls into a hole and meets a mysterious woman. He wakes up in the hospital and apparently just hit his head. At school, he joins the Knitting Club, whose only other member is Harumi Sakuraba, his upperclassman. His reason is to "do something for someone". When he comes back to his apartment, he finds Sanae Higashihongan, a ghost claiming that room 106 is her house and wanting him out but he keeps her at bay with sacred items from his grandma. They are interrupted in turn by Yurika Nijino, a magical girl who comes crashing through the window, Kiriha Kurano from underground, and Theiamillis Gre Fortorthe along with her servant, Ruthkhania Nye Pardomshiha who come out of the wall. Fighting ensues until the landlady, Shizuka Kasagi, comes in and forces them to sign a peace treaty.

Corona House's landlady, Shizuka, imposes the Corona Convention on the lot, which causes the invaders to rethink their strategies. Meanwhile, Koutarou participates in the Knitting Society, where its president, Sakuraba Harumi and Koutarou spend their time, away from troubles, but then there's word that four new students will be transferring into his school...

The lot continue to fight over the room, and eventually decide to use the inter-club obstacle course marathon as their next battleground. Koutarou and Harumi enter as part of the Knitting Society, along with Sanae. Theia and Kiriha get their underlings and make their preparations.

The fight over room 106 is officially on a cease-fire as they've all received a voucher to travel to an ocean-side resort for relaxation and hot springs. Everyone is excited and they all decide to go buy some swimsuits, but something seems to be going on with Sanae.

Sanae is caught by Ghost Hunters and almost sold to someone else.

Koutarou and the others fight over their points to try to come up with a idea for the theater club. Theia's the "Silver Princess and Blue Knight" gets picked. Sakuraba Harumi gets picked for the Silver Princess role, and practices with Koutarou, when Theia sees the real princess and knight within them. Meanwhile, Yurika brings a certain something into the apartment...

No one believes Yurika that she's a magical girl. All of a sudden, an evil magical girl, Dark Navy Maki, shows up and transfers into their class. She tries to tell everyone that Cornoa Apt 106 is in danger, but no one listens. Maki is invited to the room for a welcoming party. Yurika decides to fight all by herself to protect everyone after talking to Harumi...

Yurika protected Harumi, but fell instead. Koutarou takes a sword from Harumi after a strange light engulfs her, but Maki disappears instead. Meanwhile, Theia and Kiriha can't get a hold of them, so they go searching for them. Yurika tells Koutarou about why she became a magical girl, and they decide to head back towards Apt 106. Maki, however, waits for them there...

After a request from Kiriha, the gang helps out with a local kindergarten hero show. The Sun Squad shows up all of a sudden joining them in the mix. The show ends without incident, but Kiriha disappears and helps with cleaning up the town. Koutarou watches the girls who want to take over the town, but finally tells Kiriha something that's been on his mind. Isn't Kiriha too kind to try to take over the surface?

Kiriha invites Koutarou to an amusement park that she had been to before. Sanae and the other trails the couple as they spot a rhinoceros beetle hero show. Ruth disappears and wacky highjinks ensue. Kirito and Koutarou get on a Ferris wheel, where Koutarou hears the true purpose of her invasion and the story of her first love, but then earth starts to rumble...

It's December, and they've finally all gotten used to living each other in Apt 106 at Corona House. Koutarou suddenly starts a secret part-time job, even though he's about to start practicing for the second half of the Blue Knight play. The girls all wonder if he got a girlfriend, and start getting jealous. Just what will happen?!