In an incredibly huge highschool, there's this treasurer in the student council who manages the excessive budget for all the clubs in it. She's a bit like a cat, always spending all her time in the student council room surrounded by computers, to the point where some people doubt she exists.
Our main character is a transfer student who winds up in this highschool and next to this treasurer, and when he meets the crazy girls in the student council, he's half-forced into joining as the treasurer's helper. Of course, the president recruited him because she saw some potential in him, as he starts putting actual effort for once into helping out...
Even though the novel has the word "detective" in the title (given that the treasurer herself also has the special role of detective in the school, and she cracks a couple cases in pretty cool scenes), it's not that much of a mystery story and more of a comedy, with all the charming oddballs trying to scheme around the school budget and our straight man barely keeping up with his retorts.