Full story
In 1988, right after watching his mother die of cancer in a hospital, young Peter Quill runs outside and is taken by a spaceship. He grows up with the Ravagers, space pirates led by Yondu Udonta. Twenty-six years later, Quill calls himself Star-Lord, plays old mixtapes on a Walkman and steals a mysterious orb on an abandoned planet, Morag. He barely escapes Korath, a henchman of Ronan the Accuser, a Kree fanatic hunting the orb on Thanos's orders.
On Xandar, Quill tries to sell the orb and gets jumped by Gamora, Thanos's adopted daughter, sent by Ronan. Into the mess come Rocket, a heavily armed modified raccoon, and Groot, a giant tree who can only say “I am Groot.” They want the bounty Yondu put on Quill. The Nova Corps arrests all four and ships them to the Kyln prison.
Inside is Drax the Destroyer, who wants to kill Gamora as revenge against Ronan, who murdered his family. Quill convinces him that Gamora can lead him to Ronan. Gamora reveals she plans to betray the villain and sell the orb to another buyer. With a plan from Rocket and a lot of improvising, the five break out in Quill's ship, the Milano. They head to Knowhere, a mining town inside the severed head of a dead Celestial.
There, the Collector opens the orb. Inside is an Infinity Stone, powerful enough to wipe out planets. His assistant grabs the Stone and blows the place up. Drax calls Ronan out for a fight and loses badly. Ronan takes the Stone, and Nebula, Gamora's sister, destroys Gamora's ship in space. Quill goes out without a helmet to save her, and Yondu picks them both up. Quill promises him the Stone in exchange for help. Ronan sets the Stone in his hammer, defies Thanos and heads off to destroy Xandar.
The Guardians, the Ravagers and the Nova Corps attack Ronan's ship. Gamora beats Nebula, who flees. Rocket rams a ship into Ronan's and brings it down. Groot wraps his friends in branches to protect them and is torn apart. Ronan climbs out of the wreck ready to finish Xandar, and Quill distracts him with a dance-off. Drax and Rocket break the hammer, Quill grabs the Stone, and the four join hands to share its power and disintegrate Ronan. Quill fools Yondu with a fake orb and hands the Stone to the Nova Corps. He learns he is only half human, opens his mother's last gift, a new mixtape, and the group flies off together. In a pot, a piece of Groot grows back as a dancing baby.