The full story
The Guardians are famous now. The film opens with them protecting valuable batteries for the Sovereign, a golden race of genetically engineered perfectionists. Baby Groot dances while the team fights a massive interdimensional beast. Rocket then steals the batteries they were supposed to protect.
The Sovereign attack, and the Guardians are saved by a mysterious figure: Ego, a Celestial who reveals himself as Peter Quill's father. He's been searching for his son across the galaxy, and he brings Peter, Gamora, and Drax to his planet—which is also him, because Ego is a living planet with a humanoid avatar.
Meanwhile, Rocket and Baby Groot crash-land with their prisoner Nebula. They are captured by the Ravagers, who are under contract to deliver the Guardians to the Sovereign. Yondu's crew mutinies when he refuses to hand over Peter, and multiple Ravagers are ejected into space through the ship's airlock. Yondu, Rocket, and Groot escape in a violent sequence set to "Come a Little Bit Closer."
On Ego's planet, Peter discovers he carries Celestial genes. He can manipulate matter and energy—he finally belongs somewhere. But Ego's paradise hides a nightmare. He's been seeding planets across the universe with extensions of himself, planning to terraform all life into... more Ego. And he needs Peter's power to complete the Expansion.
Ego also admits the truth about Meredith Quill. He loved her—genuinely—and that love threatened his purpose. So he gave her the brain tumor that killed her. The moment Peter learns this, he opens fire on his own father. Ego took everything from him. The man who gave Peter life also destroyed it.
The final battle is the entire team working together against a literal planet. Rocket builds a bomb. Baby Groot has to navigate Ego's core despite not understanding "right" versus "left." Yondu sacrifices himself to save Peter, giving him his only space suit in the void above Ego's exploding form. "He may have been your father, boy, but he wasn't your daddy."
At Yondu's funeral, the Ravagers return to honor him with the colors—a ceremony for heroes—and Peter recognizes what family means. Gamora and Nebula reach a fragile truce, with Nebula leaving to hunt Thanos herself. In the post-credits scene, Ayesha of the Sovereign creates "Adam," setting up future cosmic threats.