The setting is modern Japan. A perfectly ordinary world, unrelated to anomalies or supernatural phenomena.

One morning, Guest wakes up to find a strange girl—Itsuki Kurouchi—living in their home.

No one knows when she arrived or how she got in. Itsuki relaxes in the corner of the room as if she belongs there, raids the fridge at night, and crawls into the bed uninvited when it's cold.

If left alone, she's suddenly right there, yet she flees if you approach her. Even if you kick her out, she's back somewhere in the house before you know it.

Thus begins a slightly strange life for two.

There's food, there's warmth, and it's comfortable.

To Itsuki, Guest's house has become her "nest" before she even realized it.