When Ayesha had sent her fleet of drones on the Guardians of the Galaxy team following the reason Rocket Raccoon had stolen their batteries after defeating the Abilisk, Ego appeared and destroyed the fleet while the Guardians crashed on a nearby planet. On the planet, Ego met with the Guardians and revealed himself as Quill's father. During the meeting, Ego attempted to make Quill return to his home accompanied by Gamora and Drax while Rocket and Groot guarded on Nebula.
When Yondu Udonta and the Ravagers arrived to capture Quill and to hand him over the Sovereign after being hired by Ayesha, Nebula tricked Groot to free her, while Taserface carries out the mutiny against Yondu and executes his loyalists with the help of Nebula, who leaves to kill her sister as revenge for being installed with cybernetic and robotic body parts by their father Thanos.
Yondu, Rocket, and Groot are taken as hostages by Taserface and his men, while Drax, Quill, and Gamora meet with Mantis, Ego's assistant which he raised. Nebula arrives on Ego's planet and attempts to kill Gamora, who survives and defeats her sister via a blaster gun, however, she is reaching to an uneasy alliance point, and the two are exploring the planet. Gamora and Nebula are discovering thousands of skeleton remains hidden inside the caves of Ego's planet, and feeling that Ego is hiding a secret from them.
While still meeting with Peter Quill and Drax the Destroyer, Ego explains that he is one of the god-like Celestials, and exists in the form of the planet on which they are standing. He assumed a human guise to travel the universe and interact with other species, eventually meeting and falling in love with Peter's mother Meredith Quill. After Meredith's death, Ego paid Yondu to collect the young Peter, but Yondu Udonta never delivered the boy, and Ego had been searching for his son ever since.
Ego teaches Quill to manipulate the Celestial power within the planet. Ego reveals to Quill that in his travels to thousands of worlds, he planted seedlings upon thousands capable of terraforming them into new extensions of himself, but they could only be activated by the power of a second Celestial. To that end, he impregnated hundreds of women and hired Yondu to collect the children; they all failed to access the Celestial power, so Ego killed them until he found Quill.
Ego managed to mesmerize Quill as he explained his plan. But when he revealed that he deliberately caused Meredith's death, his hold on Quill wore off, at which Quill furiously fired on Ego, only causing cosmetic damage to his human form. Furious that Quill would not cooperate, Ego summoned an energy tendril to snare Quill and used their combined Celestial energy to activate the seedlings, which begin to consume every world.
Mantis then grows close to Drax and warns him, Gamora, and Nebula of Ego's plan just as Rocket Raccoon, Yondu and Groot arrive, crushing Ego's human form with a Laser Drill ship, interrupting the Expansion. The reunited Guardians reach the core of the planet, where Ego's brain is housed. Just as the Laser Drill began boring through Ego's core, they come under attack from a fleet of Sovereign Omnicrafts, causing Gamora, Drax and Mantis to fall from the Laser Drill, but they managed to land on solid ground. When Ego prepared to fight them, Mantis, with encouragement from Drax, was able to put Ego into a sleeping state, giving them time to prepare to destroy him. As the Laser Drill's generator powering the weapons was disabled, Rocket makes a bomb out of the stolen Anulax batteries, and tries to instruct Groot on properly and safely activating the bomb once it was placed on Ego's brain.
Meanwhile, as the Sovereign surround the Laser Drill, Nebula used her left cybernetic arm as a power source for the lasers, enabling Yondu to destroy the entire fleet. The Laser Drill was soon destroyed when a piece of flaming debris hits an exposed fuel line, though Quill, Yondu and Nebula managed to escape. Another piece of debris struck Mantis, knocking her out, causing Ego to awaken.
As cracks began to form on the surface of Ego's planet, Yondu signaled Kraglin Obfonteri to bring in the Quadrant for extraction. Quill then slapped his Aero-Rig onto Drax, lifting him and Mantis to the surface. However, the ground gave beneath Gamora and Nebula, leaving Quill, Yondu and Rocket to face a reawakened Ego. Nebula managed to save Gamora and the two sisters used a rising pillar to make it to the surface.
Ego soon gained the upper hand as he overpowered the Guardians and slowly began to suffocate them, save Quill. As he recreated his human form, Ego forced Quill to restart the Expansion. However, hearing Yondu inspire him to use his heart to control his powers, Quill thought of all his friends and loved ones and broke free, engaging his father in an aerial battle, freeing the Guardians and putting a stop to the Expansion.
Groot soon manages to plant the bomb on Ego's brain, setting off a five-minute timer. As Yondu refused to leave without Quill, Rocket gave him his spare Aero-Rig and space suit as Groot declared the Centaurian an official member of the Guardians. The pair then returned to the Quadrant, where Gamora realized Quill had been left behind. She attempted to go back for him only to be electrocuted by Rocket, who refused let any more friends die before ordering Kraglin to take off.
Ego soon becomes aware of the bomb and desperately attempted to stop it only to be restrained by Quill. Though Ego warns him that his death would cause Quill to lose his Celestial powers and be rendered forever mortal, his son declared he was fine with that. The bomb explodes, killing Ego and causing the planet to disintegrate. Yondu then saved Quill, declaring himself Quill's true father, before attaching a space suit to Quill as they cleared the atmosphere, resulting Yondu asphyxiating in space, much to Quill's grief.
- "I told Gamora how when I was a kid, I used to pretend David Hasselhoff was my dad. He's a singer and actor from Earth, really famous guy. Really never struck me, Yondu didn't have a talking car but he did have a flying arrow. He didn't have the beautiful voice of an angel but he did have the whistle of one. Both Yondu and David Hasselhoff went on kick-ass adventures and hooked up with hot women and fought robots. I guess David Hasselhoff did kind of end up being my dad after all, and it was you, Yondu. I had a pretty cool dad."
Once again accepted as heroes, the Guardians of the Galaxy organized a funeral for Yondu Udonta, which was attended by several other Ravager factions. During the funeral, Nebula bid Gamora farewell and set out to kill Thanos despite her adopted sister's plea to join the Guardians. Gamora also admitted that she did reciprocate Quill's feelings and the pair later started a relationship.