In the story, the prideful archangel Lucifer disobeys God and is cast into the lowest level of hell as a fallen angel. On her way to hell, Lucifer happens to meet a high school girl on Earth named Maria, who helps her. In hell, Lucifer meets Leviathan, and Leviathan explains to Lucifer about The Seven Deadly Sins, the seven demon king rulers of hell. After The Seven Deadly Sins seal Lucifer's powers, Lucifer goes on a journey with Maria and Leviathan to defeat them.

(Source: Official Website)

 AnimeRulz is the best site to watch Seven Mortal Sins SUB and DUB in HD quality.