One day, Goku and Kami were chilling on the Lookout.
Then a spaceship crashed into it.
So Kami like, "Aw jeez, I wonder if this is anyway related to the time you crashed into my Lookout in a spaceship."
But before the dynamic duo can investigate, the pod opens, and out steps a humanoid alien with battle armor, long ass hair...and a tail.
His name is Raditz, and he's Goku's brother.
Goku is freaking out, almost childishly happy as he learns the truth about his origins, about his family, his race, and he's elated to discover there are more Saiyans out there.
His elation is quickly crushed as Raditz (Who was actually a bit flustered and touched at Goku's initial reaction, but pushes this aside) reveals himself to be a massive asshole, who refuses to call Goku anything other than Kakarot, reveals that Goku was sent to Earth to eradicate it's inhabitants, and that the Saiyans are a brutal, barbaric race of warriors.
Goku is crushed.
Raditz demands that Goku leave with him, now, to rejoin him and his fellow Saiyans, but Goku refuses to leave Earth, even moreso when Raditz threatens it. Goku reveals that he is Earth's Guardian, and it doesn't matter if Raditz is his brother, he's not going to let him harm Earth's inhabitants.
But he also tries to appeal to him. Like with Piccolo, he tries to argue there's more to life than fighting, that Raditz doesn't have to live the life he's living, but can choose another one, of his own free will.
Raditz says he has no choice.
Then he kills Mr. Popo.
Shocked, enraged, grieving, Goku charges Raditz, but before he can land a punch, he flies right into Raditz' fist, which makes him cough blood and drops to the floor.
Raditz calls Goku (Well, he calls him Kakarot but that's not the point) weak and pathetic, his kindness and mercy only weaknesses. Him status as Earth's Guardian and not it's Eradicator, he calls gay.
After gloating about his own superior strength, he then threatens to kill Kami, and this is where Goku is quite literally reduced to begging and pleading for his adopted father and mentor's life.
Out of desperation, he reveals the Dragon Balls, and how if Kami dies, they can't be used.
Raditz kidnaps Kami (He literally just chucks him into his pod, which is still crashed on the Lookout) and orders Goku to redeem himself and be deemed worthy to rejoin the Saiyans...by eradicate humanity.
And if not, then Raditz will.
Goku, naturally, is like, "No, nigga." (I'm allowed to say that, right?) and goes to the Z-Fighters Group Chat. Piccolo is the first one to respond by telling them to stop inviting him to the group chats, but Goku swiftly reveals that his evil alien brother from Space has kidnapped Kami, knows about the Dragon Balls, and wants to eradicate all of humanity, so Piccolo and the rest of the Z-Fighters, naturally, are like, "Oh shit."
And so they all go to Kami's Lookout.
Raditz uses his scouter to check out their power levels before (Very loudly) laughing and mocking them about their power levels, and bragging about how his power level is bigger than all of their's.
The rest of the Z-Fighters are like "It's not about the size of the power level, it's how you use it!"
And so an innuendo establishes one of the major themes and conflicts of this entire story.
Raditz seeks to prove that his naturally superior and larger power level outclasses skill and training and teamwork and all that gay ass bullshit, and he does so by beating the shit out of them. Goku and Piccolo are the most powerful, and if they worked together with everyone else, they could probably hold their own against him...but they don't, because they don't know how to teamwork, so they can't make the dreamwork.
But the human Z-Fighters are able to use their teamwork to at least distract Raditz long enough for Goku to power up a Kamehameha (Because he knows it, okay? Hmm, maybe I should give him some original or new attacks...) which, to everyone's (Including Goku's) shock - and Raditz's horror - actually temporarily makes Goku's power level higher than Raditz.
Here, they learn the truth about Raditz - he relies on his natural power, but unlike the rest of the Z-Fighters, has no idea how to raise or concentrate it into an attack.
Knowing that all they need is one good attack, the Z-Fighters are finally able to formulate a strategy to kick the shit out of Raditz, Solar Flare him, Goku to grab his tail, and Piccolo to fucking drill a hole in him with Special Beam Cannon.
Oh, and Goku refuses to let Raditz's tail go. That's what you get for threatening Earth. Guardian has to do what a Guardian has to do.
Except that's not the case.
Goku gets onto his knees, puts his hands over Raditz, and begins to heal him with ancient and mystical magic (It's literally Dende's Healing Magic except cooler and more mystical) and it is a severely time consuming and tiring process, but as Raditz no longer has a hole in his chest, Raditz doesn't complain. A lot.
So Goku grabs his tail, and Raditz no longer complains.
Raditz does, however, ask why Goku spared him, and Goku does his "I'm a Good Guy/You're my brother/Second chance/redemption" thing, and Raditz is stunned... but he nonetheless thanks him.
Vegeta and Nappa, through his scouter, swiftly turn on Raditz, calling him a pathetic, weak turncoat, and that they're coming to Earth to get the Dragon Balls and finally wipe out the Low-Class Trash that pollutes the purity of the Saiyan Race.
Raditz desperately pleads for them not to, and on the other side, Nappa seems to hesitate, but Vegeta's word is final.
They're coming to Earth.
Naturally, Raditz has a massive mental breakdown and laments that they're all going to die, but Goku bitchslaps him (Or grabs his tail again, whichever is more effective) and tells Raditz to quit being a little bitch, to have his redemption arc, and to believe in the power of team work and friendship.
Raditz once again cries out how they're all going to die, how Nappa has a power level of 6,000 and Vegeta one of 18,000, and that they have no possible hope of defeating them.
Goku points out how they were able to beat Raditz in their first battle with him, correctly pointing out that Power Level isn't everything. Goku offers to teach Raditz how to get stronger, and more importantly, more skilled, as long as he agrees to leave his old life behind and begin a new one, a better one.
He let's go of Raditz's tail and offers him his hand.
And Raditz takes it.