Flandre Scarlet is a magical girl vampire who lives in the Scarlet Devil Mansion. The mistress of the mansion, Remilia Scarlet, is her older sister. Apparently, Flandre going outside would cause problems, so she has been trapped within the mansion for 495 years. However, considering how powerful Flandre is, it's hard to believe anyone could stop her from going outside if she really wanted to.

Patchouli Knowledge is a sickly magician who lives in the Scarlet Devil Mansion. She can almost always be found down in the mansion's richly furnished underground library. The mistress tasked Patchouli with keeping Flandre occupied so she wouldn't leave the mansion. So, today, she is dutifully carrying out this task while reading a book.

Flandre doesn't hate reading, but she definitely doesn't enjoy it as much as the magician sitting beside her. But she can hardly be blamed for that. No one could match the pace that this insatiable bookworm happily chews through books. Having grown tired of reading, Flandre decided to play a little prank. She took one of the cranberries she'd brought out of her room to snack on and quietly dropped it above Patchouli's tea. Without looking up from her book, Patchouli deftly moved her cup out of the way. Drats.

Flandre broke the silence with a question, "I like reading, so I can endure staying cooped up inside all the time without getting that bored. But you spend way more time reading than I do, Patchouli. Isn't that kinda strange?" Patchouli showed no hesitancy in her answer. "You're the one who's kinda strange. I don't like going outside because I have a weak constitution, and it's hardly uncommon for magicians to like reading books. And besides―"

"And besides, it's mostly thanks to you that I get to read as much as I want. Since I've been tasked with keeping you down here in the basement, I can use that as an excuse to never leave the library. It's wonderful." Taking a short pause for breath, Patchouli continued, "You staying down here is good for you, good for the Scarlet Devil Mansion, and it's good for the Human Village. The way you help people while being locked in here with no way to leave is like the opposite of a locked-room murder mystery. You're a locked-room savior." "But you're basically trapped down here with me. So, what does that make you? A locked-room detective?" replied Flandre. Chuckling, Patchouli took a book from the bookcase behind her and said, "Want to read a mystery novel?" It seems Flandre wouldn't be going outside any time soon.