On the planet Alderaan, a young boy, Piotr Quell, witnesses his mother die from cancer. Upset, he runs away from home, and is abducted from Alderaan by the Ravagers, a group of space pirates led by the Chiss Yondu Udonta. Yondu is an undercover Jedi who leads space pirates who find artifacts and do tasks for the Republic and the Jedi Order. Yondu sees that the young Quell is Force-sensitive, so he brings him to the Jedi Council, who accept Yondu’s proposal to train Quell as his Padawan. Twenty years later in the year 3,688 BBY, there is a Cold War going on between the Galactic Republic and the Sith Empire. On the planet Ossus, an ancient planet which has the site of an ancient Jedi Academy, Quell steals an orb, after which Korath, a subordinate to the Sith Empire, intercepts him. Although Quell escapes with the orb, Yondu discovers his theft and issues a bounty for his capture, while the Commander of the Sith Empire, Darth Malgus, sends his apprentice, the Twilek assassin Gamora after the orb.
When Quell attempts to sell the orb on the planet Corellia, Gamora ambushes him and steals it. A fight ensues, with Quell using his special green lightsaber(which doubles as a blaster) to block Gamora’s attacks with her purple lightsaber. The fight also draws in a pair of bounty hunters: the Chadra-Fan Rokkat, and the Trandonshan Gnut. Republic forces arrives and arrest all four, imprisoning them in the Kyln. While there, an imprisoned Zabrak Jedi, Drax Tunuz, attempts to kill Gamora due to her association with Darth Malgus, who killed his family. Quell convinces Drax that Gamora can bring Ryun to him. Gamora reveals that she has betrayed Malgus, unwilling to let him use the orb’s power to conquer the Galaxy for the Sith Empire and possibly destroy planets. Learning that Gamora has a buyer for the orb, she, Quell, Rocket, Groot, and Drax work together to escape from the Kyln.
Elsewhere, on Dromund Kaas, Malgus meets with the Sith Emperor Vitiate to discuss Gamora’s betrayal. Accompanied by Drax, Quill’s group escapes the Kyln in his ship – the Milano – and flees to The Bronx, a remote criminal outpost in space built in the giant severed head of a Celestial. A drunken Drax summons Malgus while the rest of the group meet Gamora’s contact, the collector Taneleer 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. Suddenly, Tivan’s tormented assistant grabs the Stone, triggering an explosion that engulfs Tivan’s archive.
Darth Malgus arrives and defeats Drax in a lightsaber duel, while the others flee by ship, pursued by the Imperial Navy and Gamora’s sister Nebula. Nebula destroys Gamora’s ship, leaving her floating in space, and Malgus’ forces capture the orb. Quell contacts Yondu before following Gamora into space, giving her his helmet to survive; Yondu arrives and retrieves the pair. Rokkat, Drax, and Gnut threaten to attack Yondu’s ship to rescue them, but Quell negotiates a truce by convincing Yondu that they can recover the orb. Quell group agrees that facing Malgus means certain death, but that they cannot let him use the Infinity Stone to conquer the Galaxy for the Sith Empire. Drax tells Quell that he should join the Jedi Order, as he is strong in the force and is a good Jedi, having been trained by Yondu. Quell agrees, so Drax and Quell contact the Jedi Council on Coruscant via hologram. The Jedi Master, Satele Shan, says that he has seen Piotr Quell’s exploits, and that they will induct him into the Jedi, but Coruscant is going to be attacked by the Sith Empire and he must help. On Malgus’ flagship, the Dark Aster, Malgus embeds the Stone in his lightsaber, taking its power for himself. He contacts Emperor Vitiate, threatening to kill him after the conquest of Coruscant; hateful of the Emperor, Nebula allies with Malgus.
Near Coruscant, the Ravagers, the Republic forces, and Quell’s group confront and breach the Dark Aster. Malgus uses his ship to destroy the Republic fleet. On the Dark Aster, after Gamora defeats Nebula (who escapes), she unlocks Malgus’ chambers, but the group find themselves outmatched by his power until Rokkat crashes the Milano through the Dark Aster. The damaged Dark Aster crash-lands on Coruscant near the Senate building, with Groot sacrificing himself to shield the group. Malgus emerges from the wreck and prepares to go inside the Senate building, but Quell distracts him, and an epic lightsaber duel ensues with Quell and Drax fighting Malgus. Quell eventually cuts Malgus’ lightsaber in half, grabs the freed Stone, and with Gamora, Drax, and Rokkat sharing its burden, they use it to destroy Darth Malgus.
In the aftermath, Quell gives Yondu the stone, trusting him to give it to the Jedi Order. As the Ravagers leave Xandar, Yondu 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, have their criminal records expunged, and Quill learns that he is only half-human, his father being part of an ancient, unknown species. There is a celebration in Coruscant, and the Guardians of the Galaxy are given medals and recognized as saviours of the Republic by the Jedi and the Republic. They are commended for their efforts in stopping Darth Malgus and the Sith Empire. Meanwhile, back on the Sith homeworld of Dromund Kass, the Sith Emperor Vitiate is distraught at losing the Infinity Stone and failing to conquer Coruscant and the Republic, as well as the death of his former apprentice Darth Malgus.