Gave this a watch after binging seasons 2 and 3 on Netflix. Great original story!
ALERT: SPOILERS AHEAD. THIS FILM DOESN'T END THE SERIES!
This movie references a lot of season 3 (Revival of the Ten Commandments) so definitely finish S3 first.
We're introduced to a new race, Celestials, angelic humanoids who have a pair of angel wings on their backs who live on pieces of land floating high in the atmosphere. They appear to have a strict patriarchal warrior tradition, with powers resembling those of druids and godesses, including healing and the ability Ark. They believe, base don rumors passed down, that earth was lost to the Demon clan during the Holy War, and have continued to periodically fight demons since the end of the war.
We also meet a new set of antagonists, the Six Knights of Black, powerful demons said to rival even the Ten Commandments. Despite this claim, it appears they are no more powerful than the new generation holy knights in season one, as they are each easily taken down by one or two members of the Sins in the ending battle, with the excepton of their leader, who held a grudge against Meliodas. The real fight is against the Indura, a dark creature of incredible power that feeds on souls. In a twist, the creature is ultimately defeated by Hawk Mama (revealed in the credits to be the legendary porcine 'Oshiro', or Mother of Chaos), the winged sword of the Celestials, and the combined attacked "Seven Deadly sins."
If this film is considered canon, we can assume a few things:
1-Diane regained her memories once again
2-the remaining Ten Commandments want nothing to do with the knights of black
3-Gowther has no desire to return to his evil origins (for now)
4-Elizabeth is gaining control of her goddess power
It wil likely be a long wait til season 4, but this film defnitely makes up for it.