Synopsis
Peter Quill, Star-Lord, is in a Star Wars cantina chatting up a Kree woman when a troop of troopers herald the arrival of his father Jason, King of Spartax. Jason wants Peter to stay away from Earth. Which was where Peter was born after then-Prince Jason crashed his spaceship there and had an affair with Peter's mom (see #0.1 for more details).
Jason tells Peter that a council of galactic empires met and decided to make Earth off-limits, to give us a chance to evolve into our own galactic presence. Peter accuses his father of making Earth a target for the enemies of the council members, such as the Badoon.
Jason really wants Peter to come to Spartax and take up his position as heir and Star-Lord, instead of roving the galaxy with his companions. But Quill is determined never to become part of the Spartax empire.
The argument is interrupted when Gamora fights her way through the ring of troopers, thinking Star-Lord needs rescuing. Jason knows who she is, the most dangerous woman in the universe, as well as the rest of his son's gang, He's also kept track of Peter's adventures, including Thanos, the Cancerverse and a return from the dead. Star-Lord and Gamora leave.
We now skip to Iron Man on his way out of the Solar System (flying past Mars I think), as recommended by Pater Quill. But he doesn't get far before running into a Badoon spaceship heading for Earth. They engage in a space battle, and suddenly another craft appears. And the Guardians of the Galaxy join the fight. (As well as Star-Lord and Gamora there's Drax the Destroyer, Groot and Rocket Racoon.)
Drax attacks the outside of the Badoon ship. Rocket blasts a hole in it, which Groot rampages through, followed by Gamora. Star-Lord, Rocket and Iron Man combat Badoon fighter ships.
The Badoon are losing. The commander sends a message back to the Brotherhood of Badoon. Then he launches a smaller ship, and leaves the main ship to explode.
The blast damages Shellhead's systems and knocks him out. Quill takes him back to the Guardians' ship. Drax goes to help Gamora, but she fights her way out of the wreckage anyway. Rocket retrieves a fragment of Groot, which will be enough to regrow him.
Gamora figures that Jason expected Peter to come to the aid of Earth, and face great odds. She thinks that he thinks that if Quill won't do what he wants then it's better he dies, and takes Earth with him!
The Guardians follow the surviving small Badoon ships to London.