A nameless otaku (which roughly corresponds to the English geek, albeit with a focused "nerdiness" in all things anime) encounters an aggressively-looking Santa Claus, who offers to test his knowledge; if he succeeds, he can have his way with a few magically summoned beautiful girls...
Santa-kun is a quiz simulation. Santa asks a question, and the player must choose the correct answer among the three available ones. The game consists of several "matches"; in the beginning of each, a fully dressed young girl is introduced. A match consists of three rounds (25 questions all together); after each round, the girl removes part of her clothing. Upon successful completion of a match, the girl is shown fully naked.
The questions revolve (with a few exceptions) around video games and computer technology.