Two years ago, Shibuya was ravaged by a biological terrorist attack using the deadly Ua virus. Maria Osawa was saved when her father inoculated her against the virus, but is left with partial amnesia from the shock. She works as a cameraman when she reunites with a girl named Canaan in Shanghai, China. A gifted assassin with synesthesia, she is under orders from an unknown organization for a yet unspecified mission.
When the Majin awoke from the depths of darkness, the deity of fate chose six heroes and bequeathed them with the power to save the world. Adlet, a boy who proclaims himself the strongest on Earth, was selected among the Rokka no Yūsha (Braves of the Six Flowers), and he goes to the rendezvous point — but seven have gathered there. The heroes suspect that someone among the seven is the enemy, and the initial suspicion falls on Adlet.
Set in the Shinigami technical school for weapon meisters, the series revolves around 3 groups of each a weapon meister and a human weapon. Trying to make the latter a "Death Scythe" and thus fit for use by the Shinigami, they must collect the souls of 99 evil humans and 1 witch.
Valkyrie, Kazuto, Akina, Hydra and friends are back for more when the Key of time suddenly appears along with many other mysterious happenings. All seemed to be linked to a person named 'Valkyrie Ghost' who's only goal is stealing Kazuto for herself. As they team up against this seeming unbeatable foe they dig deeper into the mysterious past of Valkyrie's home planet, Valhalla.
The third season doesn't have a particular story behind everything (like "Valkyrie Ghost" in the second season). It just tells 6 stories about the life of Kazuto & Co. with some additions: Raine and Chorus get a house on earth, the last three Valhalla princesses each get an episode, and in the last episode Kazuto and Valkyrie, who in the same episode managed to stay adult, try to marry again.