舞台会设定在自横滨大街陷入混乱的那个事件以后的3个月，主人公之一的紫携手失忆少女初，再度以侦探的身份开启了他们的活动。究竟亚特为什么会把枪口对准奈斯？包裹着谜团的真相即将在今年夏天揭晓。

The Iwatobi Swimming Club is back and struggles with the recruitment of new members. At the same time, senior members Haru and Makoto have to figure out what to do after high school, challenging their love for swimming. Meanwhile an old childhood friend of Rin returns and stirs up some trouble - especially with Haru. How is the Iwatobi gang going to respond?!