Ok spoiler alert (full one) from what I've gathered.
Billis awakens to fight a ssj god, he was foretold about. He seeks out King Kai to locate this being as he heard he was his student. King Kai calls for Goku. Goku shows him the ssj stages 1-3 and tries to fight him, Billis plucks his ass away, and then decides to seek out the other saiyans, namely the prince. King Kai warns Vegeta.
Vegeta tries to suck up to him by dancing, bowing, the like and keep him calm at the party, where Vegeta was hosting the bingo tourney, Piccolo was singing karaoke, and Gohan was a drunk baboon that gets Videl shot because he's Saiyaman and demanded they shoot t him so he can pluck the bullets away. Dende has to heal Videl who gets shot and while healing her he finds she is pregnant, and Videl has him promise not to tell anyone including Gohan. Billis wants a piece of food, but Buu eats it, those two get into it, and Billis gets tired of all this shit.
They try to fight him and he manhandles them all including Ultimate Gohan and Gotenks. Billis adds insult to injury when he bitch slaps Bulma. Vegeta loses it and he taps into his own hidden power for the first time in the series as a ssj2, and it surpasses what Goku was capable of as a ssj3, as that is the first time he shows and feels true human emotion like Goku and Gohan. Goku secretly watches the fight and comments that Vegeta has surpassed him much to his own surprise. It isn't enough though. The gang ask shenron how to become a ssj god.
Shenron makes the aware of the prophecy long before there was a planet Vegeta. Aka back in planet saiya before the Tuffles and Saiyans fought over the world Vegetas dad ruled, there was a legend of a ssj god. It requires 6 pure hearted Saiyans. Back then there were only a handfuls on the original Saiyan home world and none as strong as we are used too now, or even like Bardock and King Vegeta. The first ssj god was created, but he couldn't hold the form nor harness ssj power like the current group, so he and the other 5 pure ones was defeated by the evil Saiyans on the planet, aka the twisted legend of the home world being destroyed to ruins that Vegeta describes watching Goku at the Ginyu Force.
Anyhow the gang has to figure out how they can get a 6th, let alone a sixth with ssj power within them to make Goku transform. They think about Tarble but he ain't super so that doesn't work, and that's where Pan comes into it. There's a ritual, and this ritual requires them to all go super, which is why Gohan transforms as it pulls the Saiyan out him over the Kai magic. Pans fetus is so powerful it makes the ritual work, meaning she is set to be born and make the likes of Broly and the boys be a joke. Goku undergoes a transformation, red eyes, red hair, fire aura (resembles kaioken without the muscle overload and stamina issues for a reason as King Kai based said technique off that legendary power which is why he could never do it himself and the others could never be taught it)
Goku fights Billis in the form and it wears off, but him being Goku allows him to channel that same god power into his ssj form, MSSJ, like when he fought cell and use it from then on. They fight evenly for what's described as a war, and then Billis beats Goku. Billis decides to destroy the planet because he can't have them getting stronger with Goku being a god now and Vegeta having shown that power, but when he goes to he can't muster the energy either. In a fit he tells them there are 12 universes meaning Trunks and Cell didnt come from the future but Bulma accidentally made a dimension hopper, and he's only one of 12 gods of destruction, the weakest and that Wiss is the master and stronger than him.
Wiss karate chops Billis for leaking said info, and said the chop will lead to him sleeping for 3 years for a possible return. Vegeta in retort declares that he will be the one becomjng a ssjgod next time when the time comes and wont lose.(Its said that they weren't done. And basically done in a way that can lead to series continuation and a need for training.)
And in a play on GT, pilaf fucked up a wish and he and his gang got turned into kids, the irony. And I guess it's a relevant tidbit, its said Gohan ages more like a human while Goten and Trunks age more like Saiyans thus the not growing (weird as Gohan is also noted as the only hybrid to ever be born with a tail)
Theres the full spoilers I've read and summarized from different places for you.
* they also relased a guidebook, program with the movie and at the end when it talks of the conclusions it simply states "And More!"
