This is the arc with the most divergences from GT canon. Be warned.
Things have settled after the Bebi stuff, and Pan is going to school again. Everything seems fine, and there doesn't seem to be any issues on the horizon, letting Trunks go back to learning how to business, Goten working for Capsule Corps while being kinda uncomfortable about hanger-on girls, Oob is with Goku helping him train to achieve Super Oozaru, and everyone else is just living their lives while being bothered about the Ki business(Gohan let slip that Ki could be taught to people, and that he was writing a book about it).
Then things get weird. Namely, Raditz shows up and begins blowing up the city Pan was going to school in, and when she confronts him he reveals an odd amount of strength - somehow, he's able to go Super Saiyan, but the aura is weird and corruptedly thick and red instead of gold - and manages to escape Pan. Reports of superpowered ghosts showing up all over the place, just making a mess of things. Thus, Goku contacts the Kais, and learns what's going on.
Namely, some dummy manning the machine that purified the evil from souls in Hell did something, causing it to explode right when it was processing a certain batch of souls. These souls, now regaining their bodies, proceeded to break a bunch of dangerous villains out of Hell, before portalling them to Earth for some unknown reason. And the leader seems to be an empowered Frieza, of all people - with Cell also having escaped!
The Dragon Warriors decide to split up, with different targets to find and deal with. Goku and Oob are to find Frieza, #18, #17 and Gohan are to find Cell, Goten, Trunks, and the rest are going to deal with the random other warriors who escaped... and Pan is to stay out of this, because she's just a child and this sort of violence isn't something she's prepared to deal with.
Vegeta ends up finding Nappa, like in canon. Here, he attempts to reason with him and asks why he's working with Frieza, before Nappa reveals he isn't - he and Raditz aren't serving that great enemy of the Saiyans, but instead someone else. He then accuses Vegeta of being a traitor, and goes Super Saiyan and attacks. Despite Vegeta's efforts, he's forced to kill Nappa again, and for a bit ruminates on how he killed him the first time.
Gohan, 17, and 18 encounter Cell at the place the Cell Games happened. What follows is a team fight, with all three of them tag teaming a powered-up Cell and managing to win.
Meanwhile, Raditz shows up to explicitly kidnap Pan. For some reason, he's somehow stronger, and references a "Source" to this invasion who's giving him orders. Pan is nearly overwhelmed, until Piccolo shows up to help, referencing a "Weird Hooded Guy" who also went to meet Goten and Trunks. Speaking of, their patrol dealing with all the random trash encounters this hooded guy, who despite his suspicious look gives Goten and Trunks an oddly reassuring feeling. He transports them to Pan and helps them defeat Raditz, before saying he can help them get to the source of this. When asked who he is, he removes his hood, revealing a rather Vegeta-like face, and gives a simple name:
Yamoshi.
Meanwhile, Goku and Oob encounter Frieza, after tearing through his force. When asked, Frieza reveals he's not sure who's behind this, but he'd thank them all the same - for after training for years in Hell, he discovered a way to surpass himself. He and Goku fight,, and while Frieza is strong Goku doesn't need to go beyond Super Saiyan to equal him. And then, Frieza states "You might not know this, but I'm a being capable of transforming. Most of my forms exist to regulate my power, however... but after seeing your own transformations, that filthy 'Super Saiyan', I just had to answer it in my own way. Behold, the results of my training in Hell!"
"I decided to make it gold, to show my position atop the pecking order. Hmm... let's keep the name simple. This... is
Golden Frieza."
Goku's Super Saiyan 3 strength is no match for Golden Frieza's immense power. However, at a pre-arranged signal, Oob uses a spell to make Frieza's head appear like a shining moon... and Goku transforms into a Golden Oozaru. This time, however, he... pauses, and shrinks, and soon resolves into a Super Oozaru with yellow eyes. His power in this state allows him to, while not exceed Golden Frieza's strength, at least stand up to it, and Oob lends his assistance now. However, even together, they're still losing, and Goku is having issues with controlling his form and not reverting back into the Golden Oozaru state. This is when Vegeta shows up, and his added power allows the three fighters to overcome Frieza - though as he dies, he tells them of somethign worse coming, especially for Vegeta, and that he'd savor their pain from Hell.
Meanwhile, Goten, Trunks and Pan go on a Vision Quest, guided by Yamoshi as he tells them the story of what really happened, regarding the Super Saiyan and the Super Saiyan God. He shows them Saiyan society throughout the ages, and how in times of great evil, pure-hearted Saiyans would be born to guide Saiyan society back toward a more righteous path, in case they'd be destroyed. He had been one of the later ones, and he and his companions had stood against the evil Saiyans of his time - pooling their power together into him, so that he could become the Super Saiyan God.
He then tells them that the source of this outbreak is distinctly Saiyan, even as a malice-filled presence washes over Earth. And as Vegeta and Goku turn, they behold a man in royal garb, with hair recognizable to anyone who knows Vegeta and an aura of supreme evil energy.
King Vegeta.
He accuses the Saiyans before him of turning fro the true path of the Saiyans - the path of destruction - and that he would destroy them for that, before going out to rebuild the Saiyans with all the malice he'd absorbed from every Saiyan soul in Hell. hat follows is a desperate fight, as King Vegeta wields the strength of millions of souls as his own - all the Saiyans enslaved by their own sins to his will. Despite Gohan and everyone else showing up, Vegeta remains an overwhelming force who utterly devastates a good chunk of the planet.
Meanwhile, Yamoshi finishes his story about how he failed, and that the evil within the Saiyans had built up to this point and needed to be cleared. He then tells Goten, Trunks and Pan that he was leaving the future to them, and sends them back. They arrive to the fight, and their normal Super Saiyan states are unable to help much... but then Trunks remembers Gotenks. As Oob, Piccolo, the Androids and others hold off King Vegeta, the Saiyans gather and pool their energy into Goten and Trunks. With both filled up to the brim, the others join in the fight while Goten and Trunks move apart.
"Fuuuuu...Sion! HA!"
And thus, even as the King moves to strike down Pan... a hand wreathed in scarlet flames catches his attack. Gotenks stands there, hair and eyes scarlet - the Super Saiyan God. The battle that follows is... uneven, as the evil used by King Vegeta to enslave his race cannot stand against the power of a Super Saiyan God. However, Gotenks spends too much time messing around and trying to deal with way he'd enslaved those souls, and thus ends up defusing. All hope seems lost...
Until Trunks blocks one of King Vegeta's attacks. He and Goten still retain some of the Godly power, somehow. And with everyone's combined efforts, and the heavily weakened King Vegeta losing his grip, he is obliterated, and the souls are allowed to move on into the cycle of reincarnation.