You can just read the wiki-description. It explains the entire movie.
Just so you know, this post will contain spoilers, so read at your own discretion.
Basically, Bills is the God of Destruction who destroys planets based on his own judgment so the Gods of creation can make new ones. He is awakened by his Butler/assistant Whis who is also his martial arts master. He is told of Goku and how he defeated Frieza etc... He goes to see Goku, fodderizes him in SSJ3 and goes to Earth to look for more Saiyans on info about the Super Saiyan God. He crashes Bulma's birthday party, but stays for the food. Majin Buu subsequently takes all the food that Bills wanted and Bills gets angered and destroys him. Everyone attacks Bills and he fodderizes them. Goku comes, gets into SSjG form, fights equally with Bills, but is forced to forfeit by Bills. Bills says he will now destroy earth but it was only a bluff because he didn't have the stamina to do anything more and shot a mere ki blast.
He leaves the planet and says when he comes back he better have some custards ready for him xd.
Also, Whis offers the Job of God of Destruction to Goku once Bills perishes, but Goku declines (Goku in SSJG has achieved God status equivalent to Bills hence why he got offered it).
Vegeta says the next time something like that happens he will be the Super Saiyan God! (Which I doubt because we all know Goku will retain the Strongest Saiyan title).
In hindsight, Bills ends up being sympathetic. He is the strongest being in the entire show only contested by SSJG Goku (And if you count Vegito, but he's two people).
The creators did say if the movie is successful enough, the chances of a new series being made go up, but it is not concrete. Also, it wouldn't be a sequel to GT because this movie is canon and automatically disregards SSJ4 which disregards GT as a whole. Even the creator of Dragon Ball said GT wasn't canon and this movie is.