From MangaHelpers:

Tsubaki Tenji seems like a normal high school boy, but at night he secretly works as a detective! Though he pretends to be kind of dim and clumsy at school, he's very physically skilled, and he fights using 500 yen coins as projectiles. However, he's not working just because he wants to-- he's got to pay off his vanished father's ridiculously large debt. He works for the Muroto Investigative Agency, under Muroto Shino, and alongside Shino's teenage sister, Yuki. He's always sleepy during the day from working at night, but if the school finds out the truth he could end up expelled.

Unfortunately, his classmate Tachibana Kirin caught on to his act. She finds Tenji's true skills to be incredibly cool, and his work to be very exciting. As the price for her silence, she convinces the group to take her on as Tenji's assistant detective. Now she and Tenji are in this dangerous business together! [tethysdust]