Yuuma and his girlfriend Hiromi. Yamato and his girlfriend Himari. They've been friends for years. One day while staying at their friends Yuuma and Hiromi catch Himari and Yamato having sex so Himari suggests having sex together where they can watch each other. Next, to add excitement she suggests swapping partners. However when it turns out that Yamato is far better at satisfying his girlfriend Hiromi than he is, Yuuma feels inadequate as a lover.