Masaki and Erina, childhood friends, had planned to go to the beach during spring break, and Tetsu and Shinobu decided to join them. When they arrived at the breathtaking private beach, both couples enjoyed themselves playing in the sea and on the shore. Before long, while napping on the beach, Tetsu and Shinobu secretly began having sex. But when they glanced to the side, they realized that Masaki and Erina had witnessed the entire scene. Casting aside all shame and restraint, the two couples then began to expose their twisted desires to each other, marking the start of a sex battle.