A Couple of Cuckoos 13

The Megurogawa Academy culture festival has begun! Amidst their nervousness for the M-1 competition, Nagi and Erika walk around the school together. If they lose, their cohabitation will end… What’s the “bomb” they’ve prepared to ensure their victory?! What is Soichiro planning?! And what’s the answer that Nagi has reached?!