"I" (Boku) love mathematics. Just after the high school entrance ceremony, "I" meet a beautiful girl: Miruka. Miruka is a mathematical genius. She gives me many math problems; she shows me many elegant solutions. Miruka and I spend a long time discussing math in the school library.

One year later, I meet another mathematical girl: Tetra. Tetra is one year younger than me, and asks me to teach her math. While I teach her, she begins to understand math and to love its elegance gradually.

In this third volume (series), we talk about logic puzzles, Peano arithmetic, epsilon-delta, Cantor's diagonal argument, Hilbert's program and Gödel's Incompleteness Theorems.