Phi Brain: Saigo no Puzzle

Summary

Daimon Kaito is a high school boy, who loves puzzles. One day, the president of the student council gives him a mobile device and it leads him to an underground ruin buried in the school. He challenges an "insolvable puzzle" hidden in the ruin with his girlfriend Nonoha, but the puzzle turns out to be the dangerous "The Puzzle of Sage" created by the black society "POG". He manages to solve the puzzle but it's the beginning of a puzzle battle against POG.