In 2014, Peter Quill, Gamora, Drax, Rocket, and Baby Groot are renowned as the Guardians of the Galaxy. High Priestess Ayesha of the Sovereign race has the Guardians protect valuable Anulax batteries from an Abilisk, an inter-dimensional monster, in exchange for Nebula, Gamora’s estranged sister, who was caught attempting to steal the batteries. After Rocket steals the batteries for himself, the Sovereign attacks the Guardians’ ship with a fleet of drones. A mysterious figure destroys the drones, and the Guardians crash-land on the nearby planet Berhert. The figure reveals himself as Ego, Quill’s father, and he introduces Mantis, his naïve empath servant. Ego invites Quill, Gamora, and Drax to his home planet, while Rocket and Groot remain behind to repair the ship and watch Nebula.

Ayesha hires Yondu Udonta and his crew, who have been exiled from the greater Ravager community by Ravager leader Stakar Ogord for child trafficking, to recapture the Guardians. They capture Rocket, but when Yondu hesitates to turn over Quill, whom he raised, his lieutenants, Kraglin Obfonteri and Taserface, lead a mutiny, aided by Nebula, who disables Yondu. Taserface executes Yondu’s loyalists and imprisons him and Rocket aboard Yondu’s ship. Nebula leaves to find and kill Gamora, whom she blames for the torture inflicted on her by Thanos, their adoptive father. Kraglin, who is remorseful, helps Groot free Rocket and Yondu. With Yondu’s new telekinetic arrow and control-fin, the group massacres the mutineers before setting the ship to self-destruct using Yondu’s arrow. Taserface informs the Sovereign before being killed by the explosion.

Ego, a god-like Celestial who manipulated the matter around his consciousness to form his “home” planet, explains that he projected a humanoid guise to travel the universe and discover a purpose, eventually falling in love with Quill’s mother Meredith. Ego hired Yondu to collect the young Quill after Meredith’s death, but the boy was never delivered, and Ego has been searching for him ever since. He teaches Quill to manipulate his inherited Celestial power, while Mantis grows close to Drax and tries to warn him of Ego’s plans. Nebula arrives at Ego’s planet and tries to kill Gamora, but the pair reconcile and reach an uneasy alliance upon discovering a cavern filled with skeletal remains. Ego brainwashes Quill and reveals that, in his travels, he planted seedlings on thousands of worlds that can terraform into new extensions of himself, but only the power of two Celestials can activate them. He adds that he impregnated countless women and hired Yondu to collect their offspring, but killed them all when they failed to inherit his Celestial power. When Ego admits that he gave Meredith the brain tumor that killed her, an infuriated Quill breaks out of his brainwashing and violently attacks him. Ego then parasitically syphoned off Quill’s energy to activate the seedlings, which begin to consume the worlds on which they are planted.

Rocket, Yondu, Groot, and Kraglin arrive, and together with Mantis, Drax, Nebula, and Gamora, they rescue Quill. The reunited Guardians travel to Ego’s brain at the planet’s core, where Yondu reveals that he kept Quill to spare him from Ego. As the Sovereign’s drones attack, Rocket makes a bomb using the stolen batteries, which Groot plants on Ego’s brain. Quill uses his newfound Celestial powers to fight Ego, distracting him long enough for the other Guardians and Mantis to escape. The bomb explodes, killing Ego and disintegrating the planet, but also draining Quill of his Celestial powers. Yondu sacrifices himself to save Quill by letting himself die in the vacuum of space. Despite having reconciled with Gamora, Nebula chooses to leave and resume her quest to kill Thanos. With Mantis as a new member, the Guardians hold a funeral for Yondu as dozens of Ravager ships arrive, having heard of Yondu’s sacrifice, and accept him as a Ravager again.

In a series of mid-and post-credit scenes, Kraglin takes up Yondu’s arrow and control-fin; Ogord reunites with his ex-teammates and fellow Ravager captains; Ayesha creates Adam, a new artificial being with whom she plans to destroy the Guardians; and Groot grows into a teenager.