The Seven Deadly Sins aid the Sky People against a powerful group of demons hellbent on resurrecting a demonic beast sealed over 3,000 years ago.

The story revolves around Haruka Nanase, a boy who has always loved to be immersed in water, and to swim in it. Before graduating from elementary school, he participated in a swimming tournament along with his fellow swimming club members, Makoto Tachibana, Nagisa Hazuki, and Rin Matsuoka. After achieving victory, each of the boys went their separate ways. Time passed, and in the middle of their uneventful high school lives Rin appears and challenges Haruka to a match, showing Haruka his overwhelming power. Not wanting it to end like this, Haruka, gathers together Makoto and Nagisa once again and brings a new member named Rei Ryugazaki to create the Iwatobi High School Swimming Club in order to defeat Rin.

Ichiban Ushiro no Dai Ma?, also known as Demon King Daimao, is a Japanese light novel series written by Shotaro Mizuki and illustrated by Souichi It?, published by Hobby Japan under their HJ Bunko imprint. The first volume was published on February 1, 2008, with a total of 11 volumes available as of December 2010. A manga adaptation began monthly serialization in the September 2008 issue of Akita Shoten's magazine Champion Red, and a 12-episode anime adaptation by Artland aired between April and June 2010 on Tokyo MX and other channels.