Short Answer:
The prequel DBU summary is
Goku and Vegeta and the others got recruited on a quest to take down The Grand Priests Sister cause she was erased with the Weak Universes before the events of DB. But she's not actually Evil and he is so he set them up. They beat him, Get a wish from the Super Shenron wish for all the Saiyans back and the events of DBUO start..
•—•
Long Answer:
This is what happened in Dragon Ball Ultimate to set up the events of Dragon Ball Ultimate Origins...
This is brought to you by CRMichShaw:
Grand Priest- "As you wish.. In the Tournament of Power, when the Universes were wished back.. It brought back all of the 12 universes.. including the 6 that were erased by the Omni Kings himself.."
Goku- "What's wrong with that? Weren't those universes erased because they were weak?"
Grand Priest- "Yes, the universes were weak.. But it's what leaded them that isn't. Someone even stronger than me, lays dormant inside of universe 13.. My sister, Marcinnete.."
Yamcha- "Wait? You guys have names?!"
Hiyoki- "Yes, of course.. I am the Grand Priest, but I still have a name.. It's Hiyoki.."
Gohan- "So how are we supposed to beat Marcinnete? She's stronger than you! And you're supposed to be the strongest fighter in the universe.."
Hiyoki- "I'm going to train you, for I am an angel.. Angels can't surpase their birth strength.. Even with thousands of years of training..."
Goku- "Being trained by someone even stronger than Lord Beerus and Whis! This is gonna be Great!"
Hiyoki- "Then Let's begin.."
•~©~•
Goku and the other Z Gods are pushing harder than they ever have before.. Trying to beat Marcinettes blast, who is struggling to keep up with the others..
Marcinette- "T-This isn't possible.. I'm actually losing, to mortals infact, this isn't normal... I'm being drained! What the hell!"
Goku- "KEEP PUSHING!! GRAHH!!!"
Krillin- "ALMOST- TH-THERE!!"
Krillin keeps on firing Destructo Disks into the beams pushing them forward..
Tien- "GR-GRA-GRAHH.."
Marcinette- "This is it.. I can't keep up anymore.. Im- doomed... I'm sorry A-An-Angela.. I couldn't avenge you, nor' could I save you..."
Goku- "NOW!!!"
The beams push into Marcinette, causing a giant explosion to happen.. Leaving a ginormous crater in the place of it.. With Marcinette in the middle of it.. She is laying down still conscious
Goku- "W-We won.."
Marcinette- "I-Im so-so sorry.. For everything Angela.."
Goku- "Guess we didn't need the Sun Glare Flare after all."
Vegeta- "And our Stamina is gone already too.."
Krillin- "Huff, Huff, Huff.."
???- "You mortals are pathetic.."
Goku- "Hiyoki? You're different!"
Hiyoki- "So much different, you wouldn't even believe.."
Vegeta- "What are you on about..?"
Goku- "Your eyes, they're red.."
Hiyoki- "The Omni Palace is stained that color too.."
Krillin- "W-What?"
Tien- "Why would there be blood there!?"
Hiyoki- "I would say ask Whis or the other Angels.. But they're unfortunately, expired.."
Goku- "What?!"
Marcinette- "F-Fools! I told you that he was up t-to something! Th-The evil Pr-Priest!"
Hiyoki- "Silence Sister.. You've done enough, thank you for allowing me to absorb your power.."
Goku- "Wh-What?!! Can someone please explain?!"
Hiyoki- "Idiotic Mortals. Did you not realize how weak my foolish Sister got over the time you all were fighting her? I've been slowly sucking her powers, using the God Prismarine. I also locked up all of the Angels and killed all of them.. They're all gone, and you're all doomed.."
Marcinette- "I-I told you!"
Marcinette stands up, injured holding her arm which was bloodied..
Hiyoki- "Sit down foolish Sister..."
Marcinette- "H-Ha-Haha! You're foolish, if you think I'll die like that.. GRAHH!!"
Marcinette rushes toward Hiyoki, only to be kicked in her stomach by him and sent back on the ground..
Hiyoki- "The next step, is the Omni Kings.. They will be purged, with the combined might off the three Angel Lord's.. I will rule the Universes! And I will do it wrathfully.."
Goku- "NOT ON MY WATCH! Krillin! Throw Marcinette a Senzu bean!"
Krillin- "On it!"
Krillin throws the bean towards her and tells her to eat it.. She does and then stands up and walks towards Goku
Goku- "Huff, Guess you weren't so evil after all..?"
Marcinette- "After this, you're gonna be punished.."
Goku- "Bring it on..."
•~©~•
Goku charges up into Ultra Instinct, followed by Vegeta powering up into Ultra Instinct as well.. Marcinette walks toward them, ready to fight together.. The Wind howls as Hiyoki smirks looking at them
Goku- "I'll beat that smirk off of you if I have to.."
Hiyoki- "I'll beat you never the less, hmph.. My pathetic sister fighting alongside mortals.."
Vegeta- "The mortals that she's fighting alongside are god level.. And now they will defeat you!"
Hiyoki- "Bring an army, and it'll be believable, an army of me that is.."
Marcinette- "Cocky fool.."
Hiyoki- "My whole existence has been based off of power. And now that I have it. There is only three people to get in my way..."
Goku- "Guess this is the end of the line. It's do or die time Vegeta.."
Vegeta- "Let's go.."
-Start Music-
Goku, Vegeta and Marcinette all team up against Hiyoki trading blows with him. He blocks a punch from Goku and jumps over Marcinettes ,Vegeta grabs his feet and slams him against the ground. Vegeta goes to hit him again. But is blocked Hiyoki pulls back his free hand and fires an energy blast. Blowing away the others.
Hiyoki- "You all have a motive. So I'll share mine. I've always dreaded the Omni King as I and my siblings were brought into existence. Just to serve him. It's ridiculous! Haha! If he gets all this power! Why don't us Angels rule too?! So I'll bring my own Justice and slaughter the children by myself. I SOUND CRAZY, DON'T I? DONT I?"
Goku- "You'd slay millions! You wouldn't stop at just ZENO!"
Hiyoki- "Millions that would stand in my way! STARTING WITH YOU THREE!"
Vegeta- "I won't let anyone hurt MY BULMA!"
Vegeta rushes toward Hiyoki punching him. Hiyoki punches Vegeta in his stomach. Causing him to spit up blood. They both take a step back to rush towards eachother again. Punching Hiyoki, Vegeta opens his fist on Hiyokis face and blows him away!
Vegeta- "Insignificant trash..."
Marcinette- "Tch! Behind you!"
Hiyoki teleports behind Vegeta and blasts him in his back!
Vegeta- "Gah!"
Goku teleports to Hiyoki and kicks him straight up in the air. Teleporting to him again and punching him in his stomach which is exposed. Hiyoki goes flying down where Marcinette catches him. He spins around kicking her in her face as she tries to throw him down
Hiyoki- "You're all so idiotic. To think that I'd drop my gua-"
Goku- "HAAAAAA!!"
Goku fires a Kamehameha into Hiyoki. Blowing Hiyokis GI to shreads. Hiyoki is pushed into the ground from the sheer pressure. As the Kamehameha keeps going
Goku- "IM PUTTING ALL MY ENERGY INTO THIS ONE!"
Vegeta flies to Goku chuckling as he realizes this is his last hope to defeat Hiyoki
Goku- "VEGETA! NOOOO!!"
Marcinette- "The cocky prince makes the ultimate sacrifice.."
Vegeta- "Three times I've done this. This will be the last. Kakarot. Tell my family that I love them, a Prince must put up... One Final Atonement..."
•~©~•
2 weeks have passed
Goku- "I'm sorry. ChiChi, I'll be back..!"
They fly off but stop when they hear a bang nearby
Goku- "What was th-"
Zeno- "Gui! Goku! You saved us!"
F. Zeno- "Yes! You save us!" (Future Zeno)
Goku- "AHH- Oh hey Zeno! And Zeno 2!"
Zeno- "We're in your debt!"
F. Zeno- "Anything!"
Goku- "Could you do me a favor.."
Zeno- "Yes!"
Goku- "Bring back everyone killed by Hiyoki.."
F. Zeno- "Done!"
Goku- "And could you please bring back a friend of mine.."
Zeno- "Okay!"
Goku- "Bring back.. Vegeta.. Nappa, Raditz, King Vegeta, and my father and mother. Bardock and Gine..."
F. Zeno- "Okay, I will!"
With a quick movement of his hand. Zeno brings back everyone listed there
Bardock- "K-Kakarott!?"
Gine- "Kakarott! My baby boy!"
Bardock- "Come on Gine! Get off!"
Gine- "I'm so happy!"
King Vegeta- "V-Vegeta.. Is that you!?"
Vegeta- "I told you not to revive me Kakarott.. But you knew why.."
Goku- "You wanted to atone for your sins with all the people here..."
Vegeta- "Hmph.."
Goku- "I guess this is.. Our, happy ending?"
Bardock- "I guess so son.."
Raditz- "You have alot of explaining to do.."
Nappa- "WHERE IS THE POKEMON!!!!!!!!!"
•~©~•
DBUO stands for Dragon Ball Ultimate Origins. The plot can be quite confusing, so here's the gist of it!
2 years after defeating Hiyoki, the Z gods face Omega Shenron. And the events of GT take place, but no Ssj4, only MUI. Goku is forced to kill Omega Shenron by using Kaioken ×20 MUI and firing a Kamehameha through Omega Shenrons chest.
5 years after, the Black Star Balls re surface, and Goku comes to the conclusion he must fend off the Dragon dwelling in the balls for eternity. He goes into the realm of Shenron. Fighting The Ultimate Shenron for years until the troublistic Duo of Ize and Kuro come to Earth
Izes Past:
Radi- "This child will be something Lord Frieza!"
Frieza- "HE BETTER B- GAHHH!"
The Emperor is giving birth to his son. When it is over, only one tyrant is left alive
Radi- "T-The emperor is de-"
He is struck in his heart by another solider, Christi
Christi- "The emperor's son will pay for his father's wrong doings!"
They drop the young Arcosian off at a planet with no sign of life in Universe 6
**Yawn**
???- "Lord Champa! I've found a baby on our lawn! It seems to be the Emperor Friezas son"
Champa- "What use could I have for him?"
Vados- "He has incredible potential. If we train him, he can possibly grow to be stronger than your brothers pupils!"
Champa- "Whatever to end my brothers fun with that cursed Saiyan. Train him, tell him Goku was Friezas murderer! And send him off in 4 years time! Hell, maybe even 3. I don't care! I have food to feast on!"
Present time
After Ize travels and meets Kuro. The rest is history. What happens next, well you'll just have to wait and see
Next-time on
Dragon Ball Ultimate Origins!!
The end-