You said it yourself, bitch! We're the Guardians of the Galaxy!
~ Star-Lord before defeating Ronan the Accuser.
You're right. We're family. We leave no one behind. Except maybe you.
~ Drax to Neubla.
The Guardians of the Galaxy is a band of intergalactic former outlaws and heroes who have teamed together to protect the Galaxy from disastrous threats. They managed to save the planet of Xandar from the Kree warlord Ronan and have been recognized under the Nova Empire.
They are the eponymous titular main protagonists of Guardians of the Galaxy film series and the deuteragonists of Avengers: Infinity War and Avengers: Endgame.
When Peter Quill, known as Star-Lord stole the Orb, Ronan the Accuser's Sakaarans forced hunted for the Ravager. He used the Milano to escape. Yondu Udonta and the Ravagers went to Morag after the Orb but discovered Quill's treachery and set a bounty on him.
Elsewhere in the galaxy, Thanos, The Mad Titan, his lieutenant The Other, and his daughter Nebula met with Ronan the Accuser. Ronan argued with the Other about the Orb, then killed him with his hammer and sent Gamora to retrieve it. On Xandar, Quill tried to sell the Orb to a Xandarian Barterer but he refused after hearing of Ronan. Gamora then launched a sneak attack on Quill but was intercepted by bounty hunters Rocket Raccoon and Groot. The Nova Corps then arrested them.
The group was sentenced to Kyln Prison Facility where Gamora was endangered by inmates whose families were victims of Ronan. As they tried to kill Gamora, Drax the Destroyer stopped them, scaring the other prisoners away. Drax desired to kill Gamora himself for the death of his family. Quill convinced Drax that by letting Gamora live he would get the chance to exact his vengeance on Ronan as Gamora reveals she had betrayed him. Rocket plotted an escape plan, but Groot initiated it earlier than planned. They were forced to use a makeshift plan to escape from the Kyln with the Orb. They managed to do so successfully.
After their escape, Gamora realized that the Collector knew something about the Orb. So the group traveled to Knowhere, a city inside the severed head of a Celestial. In the city, the Collector said the Orb was one of the Infinity Stones, capable of destroying worlds. The Collector's distraught servant unleashed a blast from the Orb destroying the Collector's Museum. Ronan's forced invade Knowhere thanks to Drax sending a message challenging Ronan. Nebula went after Gamora escaping in an industrial pod with the orb. Rocket and Star-Lord gave chase but Gamora was blown into space and lost the orb to Nebula.
Quill sent his coordinates to Yondu Udonta and gave Gamora his helmet until they were saved by the Ravagers. Quill promised the Orb to Yondu if they would help kill Ronan. As the group was reunited, Quill encouraged the group to join him saving Xandar from extinction. The team and the Ravagers planed an ambush on the Kree and the Dark Aster.
The Ravagers were outnumbered by the Kree until the Nova Corps helped to build a force field to contain the Dark Aster. Peter Quill, Gamora, Drax the Destroyer and Groot infiltrated the Dark Aster and fought through the Sakaarans soldiers, killing Korath the Pursuer in the process. Nebula dueled Gamora but was thrown off the ship; she hijacks a Ravager's ship and escaped. The group arrived and attacked Ronan the Accuser with the Hadron Enforcer; he was unphased, however, and easily overpowered the group until Rocket Raccoon crashed into the ship. As the ship crash landed, Groot sacrificed himself to shield the team.
Ronan raised and mocked them calling them failed guardians. Rocket rebuilt the cannon while Quill distracted Ronan allowing Rocket and Drax to destroy the hammer. Quill grabbed the Orb causing a cosmic storm. Before its power could kill him the others joined hands with him allowing Quill to withstand its power and blasted Ronan killing him and declaring themselves "Guardians of the Galaxy".
Quill gave Yondu a fake Orb container and gave the real one to the Nova Corps. With their slates cleaned, Nova Corps gave them a rebuilt Milano and told Quill he was only half-human and his other ethnicity was currently unknown. A twig retrieved by Rocket began to grow into a new Groot and the Guardians left Xandar.