1988: Peter Quill (Wyatt Oleff) sits in the waiting room of a hospital, listening with headphones to "Awesome Mix Tape No. 1" on his Walkman. His grandpa (Gregg Henry) comes out and gets him so he can say goodbye to his mom, who is dying from a brain tumor. His mom gives him a present and tells him his father was an angel, and that Peter is just like him. She asks for his hand, but he's too scared to take it. Just then, she dies. The distraught Peter runs outside and is abducted by a spaceship.
2014: On the planet Morag, Quill (Chris Pratt) searches for a mysterious orb whilst listening to "Awesome Mix Tape No. 1" on his Walkman. He finds the orb in an abandoned temple and takes it out of a laser enclosure. Almost immediately, he is ambushed by a Kree soldier, Korath the Pursuer (Djimon Hounsou) and a couple of Sakarrans. After a gunfight in which Korath's men are killed, Quill makes it to his ship, the Milano, and escapes.
As Quill leaves Morag, he gets a call from his father figure, Yondu Udonta (Michael Rooker) who heads the Ravager platoon that raised Peter, who demands Peter to give him the Orb. Peter decides he's going to sell it on his own. After Peter hangs up, Yondu's men reprimand him for always being soft on Peter despite his constant betrayals, which Yondu brushes off (even threatening one of them with his arrow), being more interested in finding out who else is interested in the Orb.
Korath returns to the Dark Aster, a Kree warship captained by a genocidal tyrant, Ronan the Accuser (Lee Pace), who seeks revenge on the Nova Empire for the deaths of his father, grandfather and great-grandfather in the thousand year war between the Nova and Kree empires that was recently ended with the signing of a peace treaty. Korath reports that he failed in getting the orb and that it's in the hands of Peter. Ronan wants the Orb because he can trade it to Thanos the Mad-Titan in exchange for Thanos destroying Xandar, capital of the Nova Empire. Ronan initially plans to send Thanos' daughter Nebula (Karen Gillan) after Peter, but her sister Gamora (Zoe Saldaña) volunteers. Over Nebula's complaints, Ronan sends Gamora.
On the planet Xandar, Peter takes the Orb to a broker (Christopher Fairbank) and asks what it is, because Ronan's goons are after it too. On hearing Ronan's name, the broker immediately reneges on the deal, not wanting anything to do with the orb, and removes Peter from the shop. Outside, Gamora ambushes Peter and tries to retrieve the Orb herself. As they fight, they are ambushed by a pair of bounty hunters: Rocket (Bradley Cooper), a talking raccoon with cybernetic implants, and Groot (Vin Diesel), a giant humanoid tree. After a drawn out fight, Peter, Gamora, Rocket, and Groot are arrested by the Nova Corps, and after processing, are thrown in the Kyln, a prison for criminals throughout the galaxy.
Pretty much everyone in the Kyln wants to kill Gamora because she's the daughter of Thanos. One of the inmates, Drax the Destroyer (Dave Bautista), has a vendetta against her because Ronan killed his wife and daughter, so he wants to avenge them by taking the life of one of Ronan's associates in return. Gamora explains that she's not related to Ronan and she was planning on double crossing him. Peter argues in Gamora's favor, and convinces Drax to work with them.
Meanwhile, Ronan is summoned to meet Thanos at his at his domain in an asteroid field. Ronan reports to Thanos (Josh Brolin) that Gamora is a traitor, and kills one of Thanos's underlings for showing disrespect to him. Thanos tells him to take care of her and get the Orb, threatening a very painful death upon him if he doesn't. He also notes that Gamora is his favorite daughter, which angers Nebula.
In the Kyln, Gamora has a plan to sell the Orb to another interested party. Peter, Rocket, and Groot agree to help and split the reward. Rocket has a plan to escape the prison. He needs one of the guard's wrist devices, a prisoner's prosthetic leg and a battery from a tall column in the prison that powers the alarms. As he explains that it's very important to take the battery last, Groot grabs it first. The alarms go off and security robots are deployed to maintain order. Gamora goes to get a wrist device as Peter negotiates for the prisoner's leg. Drax decides to join in the escape, realizing that eventually Ronan will go after Gamora and then he can kill Ronan himself.
Everyone gets the item they agreed to get, and they all make it to the guard's station near the top of the prison. Rocket is surprised that Peter actually got the leg and revealing that he only wanted it as a joke. Just as it seems that they have run out of time, Rocket disables the the prison's artificial gravity and uses the guards' robots as jets to fly out of the prison. They escape, but Peter leaves the others as he doubles back to retrieve his Walkman.
Yondu and his right-hand Kraglin (Sean Gunn) visit the broker on Xandar, and find out Peter still has the Orb. Threatening the broker with his arrow, Yondu gets the man to reveal where Peter might have traveled.
Everyone on Peter's ship gets to know one another better. They don't like each other at all. Soon they arrive at Knowhere (a space station built in the severed head of a Celestial) to meet with the Collector. While they wait for their appointment, Drax, Rocket, and Groot get drunk and gamble. Gamora and Peter bond over Peter's music. He explains that his mom made him the mix tape of her favorite songs. She listens and likes it. He asks her to dance, but she doesn't trust him. He says it reminds him of an old fable about other people who didn't dance, called Footloose. He makes a pass at her, but she quickly rejects his advances.
Before they can meet with the Collector, Drax, Rocket, and Groot get drunk and get into a big fight. Drax thinks Groot is dumb and Rocket is tired of people calling him a rodent. Peter talks them out of fighting. The Collector's assistant fetches them and they all go to meet him. Drax goes off on his own and makes a call to Ronan.
The Collector (Benicio del Toro) has a giant assortment of collectibles from all over the galaxy, including Howard the Duck, Cosmo the Soviet Space Dog. The Collector puts the orb into a kind of lathe, which unscrews the two halves of the Orb to reveal that it's a capsule for an Infinity Stone. The Collector explains that the Infinity Stones are six singularities created during the Big Bang. They can be used to wipe out entire civilizations. The Collector's assistant suddenly grabs the stone, seeking to kill him after years of being his slave. The power of the stone is too much for her to hold, and she ends up combusting in the subsequent explosion. Peter and the others manage to escape with the Orb, carrying the Infinity Stone. They decide the safest place to take the stone is the Nova Headquarters on Xandar.
Suddenly the group is ambushed by Ronan and his crew, responding to Drax's call. Yondu's Ravager platoon also shows up, having tracked Peter down from the broker. Gamora, Rocket and Quill take off in single passenger ships while Nebula and some of Ronan's goons chase after them. Drax tries to fight Ronan, but gets beaten up.
Nebula chases Gamora's ship above the atmosphere and blows it up. Nebula manages to seize the Infinity Stone and leaves Gamora floating in space to die. Peter realizes that he can't allow Gamora to die, so he calls Yondu to tell him where he is, then exits his ship, and floats to Gamora, where he gives her his gas mask to keep her alive until Yondu can beam them onto his ship.
Aboard the Dark Aster, Ronan tells Thanos that now that he (Ronan) has the Infinity Stone, he can cut Thanos out of the deal. Instead, he will destroy Xandar himself and then go after Thanos. He puts the Infinity Stone into his war hammer.
Back on Yondu's ship, Yondu contemplates killing Peter for double-crossing him, but he reconsiders when Peter says that he has a plan to get the Orb back. Meanwhile on Knowhere, Drax, Groot, and Rocket decide to join forces and save Peter and Gamora from Yondu. They fly the Milano to Yondu's ship and threaten to blow it up with a special weapon Rocket built unless Yondu releases Peter and Gamora. Peter emerges from the crowd aboard Yondu's ship, and persuades Rocket that his threats were unnecessary.
Now aboard the Milano, Peter explains that they must prevent Ronan from touching the Infinity Stone to the ground of Xandar, which will wipe out the planet, and that he has a plan to stop Ronan. The others ask Peter if he really has a plan, or if he was lying. He says he has part of a plan. After much discussion, he reminds them that they've already lost so much to Ronan that at this point they have nothing to lose. Slowly, each of them stands up and announces that he will join the fight against Ronan.
Peter warns the Nova Corps that Ronan is coming to destroy them and they should get ready, telling them that they should believe his message, because he's not "100% a dick". Rocket's idea is to blast a hole in Ronan's ship so Peter, Groot, Drax and Gamora can board it. Gamora will then cut the power for Ronan's security and then they'll blow him up with a cannon that Rocket made.
They start the plan. Rocket blows a hole in the Dark Aster with his Hadron Enforcer, and then helps Yondu and his men defend Xandar against Ronan's fighter ships. Amidst the fight, Yondu gets shot down. He's cornered by a bunch of Sakaaran necrocraft on the ground and ordered to turn on the Nova Corps. He instead uses his arrow to kill the entire platoon at once.
On the Dark Aster, Gamora engages Nebula, while Drax, Peter and Groot go to the bridge of the ship.
Dozens of Nova Corpsmen link together to form a giant net and slow the descent of the Dark Aster. Gamora finally beats Nebula, who falls, landing on a Ravager ship and commandeers it, throwing its pilot out and flying away. Ronan orders his necrocraft to kamikaze into Xandar. Rocket and Yondu's men shoot down as many of the ships down as they can.
Almost to the bridge, Drax kills Korath. Groot grows a long thin branch and thrashes a large number of Ronan's men. Eventually, everyone makes it to the bridge to fight Ronan. Again, he's too strong for them, but Peter manages to shoot Ronan with Rocket's super cannon, to no avail. Rocket crashes the Milano into the bridge and seems to have destroyed Ronan. The entire ship is plummeting towards Xandar, so Groot forms a giant nest around all of them for safety. Rocket is upset because he knows that what Groot is doing will end up killing him. Groot, who up to this point has only said "I am Groot," responds with "We are Groot."
The ship crashes, and indeed, Groot is killed, reduced to a pile of twigs. The battle is not over though, because Ronan is still alive and well. He sarcastically calls Peter and his crew the Guardians of the Galaxy. As he raises his hammer for a final blow, Quill stands up and starts to dance to the song "Ooh-ooh Child," and challenges Ronan to a dance-off. Ronan is confused, until Peter reveals it was all one big distraction, as Drax and Rocket shoot Ronan's hammer at him, releasing the Infinity Stone. Peter grabs the stone before Ronan can. Peter starts to be affected in the same way as the Collector's assistant, until Gamora tells him to take her hand. He flashes back to when he didn't have the courage to take his mom's hand, and he now grabs Gamora's. Drax and Rocket hold hands with the others, and the power from the Infinity Stone is spread among the four of them, causing a burst of energy that vaporizes Ronan. Rocket collects one of the twigs that was once Groot.
Yondu appears and demands the Stone as per his original deal with Peter. Peter hands him the Orb. As Yondu and his men take off, one of them notes that he likes Peter and he's glad they didn't take him back to his dad like they were supposed to all those years ago. Peter does so, instead slipping Yondu a fake orb containing a Troll doll, much to Yondu's amusement. Gamora is upset that Peter gave up the stone. Peter reveals that he switched the Orbs, and he still has the one with the stone.
At the headquarters for the Nova Corps, they turn over the Orb that contains the Infinity Stone and Peter learns he's only half Terran and half something ancient and unknown. They speculate that his nonhuman genes allowed him to harness the power of the Infinity Stone without dying.
Peter finally opens the present his mother gave him before she died. It's a cassette labeled "Awesome Mix Tape no. 2." He asks the Guardians where they want to go. He asks if they want something good or something bad. He then decides they should do something that is both. Rocket holds a pot in which he has stuck the stick that he retrieved after Groot's sacrifice, in which we see the face of a tiny Groot.
During the credits, baby Groot (slightly larger than in the previous scene) dances in his pot to Quill's mixtape, while a nonchalant Drax polishes his knife in the background. The baby Groot stops when Drax looks at him, but he starts again when Drax looks away.
After the credits, the Collector is seen sitting in the wreckage of his exhibits, drinking, when Cosmo comes up and licks his face. We hear a voice that says that's gross. The camera turns to reveal Howard the Duck sitting in a broken cage, drinking as well.
