In 1988, following his mother’s death, the child Peter Quill is abducted from Earth by the Ravagers, a group of alien thieves and smugglers led by Yondu Udonta. In 2014, on the abandoned planet Morag, Quill steals a mysterious Orb but is attacked by henchmen of the fanatical Kree renegade Ronan the Accuser, led by Korath. Although Quill escapes with the Orb, Yondu discovers his theft and issues a bounty for his capture, while Ronan sends the assassin Gamora after the Orb.

When Quill attempts to sell the Orb on Xandar, the capital of the Nova Empire, Gamora ambushes him and steals it. A fight ensues, drawing in a pair of bounty hunters: Rocket, a genetically and cybernetically modified raccoon, and his companion, the tree-like humanoid Groot. Nova Corps officers arrest and detain them in the Kyln prison. An inmate, Drax the Destroyer, attempts to kill Gamora due to her association with Ronan, who killed his wife and daughter. Quill convinces Drax that Gamora can bring Ronan to him, though Gamora reveals that she has betrayed Ronan, unwilling to let him use the Orb’s power. Learning that Gamora intends to sell the Orb to the Collector Taneleer Tivan, the five of them work together to escape the Kyln in Quill’s ship, the Milano.

Ronan meets with Gamora’s adoptive father, Thanos, to discuss her betrayal. Quill’s group flees to Knowhere, a city planet inhabited by thieves and merchants made from the severed head of a dead Celestial. A drunken Drax summons Ronan while the rest of the group meets Tivan. Tivan opens the Orb, revealing an Infinity Stone, an item of immeasurable power that destroys all but the most powerful beings who wield it. Tivan’s slave, Carina, grabs the Stone, triggering an explosion that engulfs Tivan’s collection. Ronan arrives and easily defeats Drax, while the others flee, pursued by Ronan’s followers, including Gamora’s adoptive sister Nebula. Nebula destroys Gamora’s ship, leaving her floating in space, and Ronan’s fighters capture the Orb.

Quill contacts Yondu before following Gamora into space, giving her his helmet to survive; Yondu arrives and retrieves the pair. Rocket, Drax, and Groot threaten to attack Yondu’s ship to rescue them, but Quill negotiates a truce, promising the Orb to Yondu. Quill’s group agrees, refusing to let Ronan use the Infinity Stone to destroy the galaxy. On Ronan’s flagship, the Dark Aster, Ronan embeds the Stone in his warhammer, taking its power for himself. He betrays Thanos, threatening to kill him after first destroying Xandar. Hateful of her adoptive father, Nebula allies with Ronan.

The Ravagers and Quill’s group join the Nova Corps to confront the Dark Aster at Xandar, with Quill’s group breaching the warship with the Milano. Ronan uses his empowered warhammer to destroy the Nova Corps fleet. Drax kills Korath, and Gamora defeats Nebula, who escapes. However, the group finds themselves outmatched by Ronan’s power. Rocket crashes a Ravager ship through the Dark Aster. The damaged Dark Aster crash-lands on Xandar, with Groot sacrificing himself to shield the group. Ronan emerges from the wreck and prepares to destroy Xandar, but Quill distracts him, allowing Drax and Rocket to destroy Ronan’s warhammer. Quill grabs the Stone, and with Gamora, Drax, and Rocket sharing its burden, uses it to vaporize Ronan.

Quill gives Yondu a container supposedly containing the Stone and gives the real one to the Nova Corps. As the Ravagers leave Xandar, Kraglin Obfonteri remarks that it turned out well that they did not deliver Quill to his father per their contract. Quill’s group, now known as the Guardians of the Galaxy, has their criminal records expunged, and Quill learns that he is only half-human, his father being part of an ancient, unknown species. Quill finally opens the last present he received from his mother, a cassette tape filled with her favorite songs. The Guardians leave in the rebuilt Milano along with a potted sapling cut from Groot, which grows into a baby version of him.

In a post-credits scene, Tivan sits in his destroyed archive with two of his living exhibits: a canine cosmonaut and an anthropomorphic duck.