The full story
In 1988, young Peter Quill watches his mother die of cancer in a Missouri hospital. Overwhelmed with grief, he runs outside—and is immediately abducted by an alien spacecraft. Twenty-six years later, Peter is an outlaw calling himself "Star-Lord," stealing artifacts across the galaxy for a group of space pirates called the Ravagers, led by the blue-skinned Yondu.
Peter recovers a mysterious orb from an abandoned planet, but he's not the only one seeking it. Korath, a soldier serving the Kree fanatic Ronan the Accuser, tries to take it. Peter escapes, but instead of delivering the orb to Yondu, he tries to sell it himself on the planet Xandar.
His attempted sale attracts three hunters: Gamora, an assassin sent by Ronan (secretly planning to betray him); Rocket, a genetically engineered raccoon bounty hunter; and Groot, Rocket's partner, a sentient tree whose vocabulary is limited to "I am Groot." Their chaotic fight gets all four arrested by the Nova Corps, Xandar's military police, and thrown into the Kyln prison.
In prison, Gamora faces execution by inmates who recognize her as Ronan's assassin—she is Thanos's adopted "daughter," taken from her home world as a child and raised as a weapon after he murdered her family. A massive prisoner named Drax the Destroyer plans to kill her as revenge for his family's murder. Peter talks him down by pointing out that Gamora planned to betray Ronan, making her a means to reach him.
The unlikely group escapes together after Gamora reveals she has a buyer for the orb: the Collector, an ancient being who hoards rare items. The orb is worth four billion units. Despite mutual distrust, they agree to split the profit. During the journey, Peter shares his most prized possession: a Walkman and mixtape of 1970s music, the last gift from his mother. Gamora begins to see him as more than a thief.
The Collector opens the orb, revealing an Infinity Stone—the Power Stone, capable of destroying entire planets. The Collector's enslaved assistant grabs it, hoping for freedom, and is instantly destroyed along with much of the Collector's archive. Ronan, alerted to their location by Drax (who drunkenly called him seeking a fight), arrives with his forces. He takes the Stone and departs to destroy Xandar, ignoring his deal with his master, Thanos. If the Stone can destroy planets, Ronan will use it himself.
Peter contacts Yondu, and the Ravagers join with the Nova Corps and the Guardians to defend Xandar. The battle is massive: Ravager ships and Nova fighters engage Ronan's fleet while the Guardians board his ship, the Dark Aster. Drax gets his confrontation with Ronan and is brutally defeated. Ronan seems unstoppable.
Rocket crashes a ship through the Dark Aster, and Groot sacrifices himself to save the others as the vessel plummets toward Xandar, wrapping them in a cocoon of his own body. "We are Groot," he says, giving his only variation on the phrase. The crash destroys Groot, leaving only a twig.
Ronan rises from the wreckage, ready to touch the Stone to Xandar's surface and annihilate its population. Peter distracts him by... challenging him to a dance-off. The absurdity gives Rocket time to shoot Ronan's warhammer, and Peter grabs the exposed Stone. The Power Stone should kill any mortal instantly, but Peter isn't entirely mortal. Gamora, Drax, and Rocket grab Peter's hand, sharing the Stone's energy. Together, they direct the power at Ronan and destroy him.
The Nova Corps takes custody of the Stone, and to the Guardians' surprise, expunges their criminal records. Peter finally opens the last present from his mother: a second mixtape called "Awesome Mix Vol. 2." And Rocket tends to a pot with a small twig—which begins to dance. Baby Groot is growing.
In the battle's aftermath, a Nova Corps officer reveals that Peter is only half Terran—his mother was of Earth, but his father is something ancient the Nova Corps have never encountered, which may explain how he survived holding the Stone.
In a post-credits scene, the Collector sits in his destroyed archive. One of his exhibits—a dog in a Soviet spacesuit and a humanoid duck—wanders past. The duck takes a drink and comments on the Collector's misfortune. It's Howard the Duck.