From MangaHelpers:
Amou Academy is an enormous educational establishment that boasts of 200,000 students and a gigantic campus that is actually its own city. Sakai Anzu is one of two members of the Detective agency operating on campus and is a normal high school girl. On the other hand, the other member, Yuri, is something out of this world. Yuri normally looks like a young girl but she is able to use sorcery to transform herself into a more mature appearance where she gains the ability to solve mysteries and crimes. There is one drawback however and it is that in that state, Yuri needs sexual energy to use her deducing skills and Anzu is there to give into her erotic needs.