Amid a galactic civil war, Rebel Alliance spies have stolen plans to the tyrannical Galactic Empire's Death Star, a massive space station capable of destroying entire planets. Imperial Senator Princess Leia Organa of Alderaan, secretly one of the Rebellion's leaders, has obtained its schematics, but her ship is intercepted by an Imperial Star Destroyer under the command of the ruthless Darth Vader. Before she is captured, Leia hides the plans in the memory system of astromech droid R2-D2, who flees in an escape pod to the nearby desert planet Tatooine alongside his companion, protocol droid C-3PO.

The droids are captured by Jawa traders, who sell them to moisture farmers Owen and Beru Lars, the guardians of their nephew Luke Skywalker. While Luke is cleaning R2-D2, he discovers a holographic recording of Leia requesting help from a former ally named Obi-Wan Kenobi. Later, R2-D2 goes missing, and while searching for him, Luke is attacked by scavenging Sand People. Luke is rescued by Obi-Wan, an aged hermit. Obi-Wan tells Luke of his days as one of the Jedi Knights, former peacekeepers of the Galactic Republic, who drew mystical abilities from a metaphysical energy field known as the Force, but were hunted to near-extinction by the Empire. Luke learns that his father fought alongside Obi-Wan as a Jedi Knight during the Clone Wars until Vader, Obi-Wan's former pupil, turned to the dark side of the Force and murdered him. Obi-Wan offers Luke his father's old lightsaber, the signature weapon of the Jedi.

R2-D2 plays Leia's full message, in which she begs Obi-Wan to take the Death Star plans to Alderaan and give them to her father, a fellow veteran, for analysis. Although Luke initially declines Obi-Wan's offer to accompany him to Alderaan and learn the ways of the Force, he is left with no choice after discovering that Imperial stormtroopers have killed his aunt and uncle and destroyed their farm in their search for the droids. Traveling to a cantina in Mos Eisley to search for transport, Luke and Obi-Wan hire Han Solo, a smuggler indebted to local mobster Jabba the Hutt. Pursued by stormtroopers, Obi-Wan, Luke, R2-D2, and C-3PO flee Tatooine with Han and his Wookiee co-pilot Chewbacca on their ship, the Millennium Falcon.

Before the Falcon can reach Alderaan, Death Star commander Grand Moff Tarkin destroys the planet after interrogating Leia for the location of the Rebel Alliance's base. Upon arrival, the Falcon is captured by the Death Star's tractor beam, but the group evades capture by hiding in the ship's smuggling compartments. As Obi-Wan leaves to disable the tractor beam, Luke persuades Han and Chewbacca to help him rescue Leia after discovering that she is scheduled to be executed. After disabling the tractor beam, Obi-Wan sacrifices himself in a lightsaber duel against Vader, allowing the rest of the group to escape the Death Star with Leia. Using a tracking device, the Empire tracks the Falcon to the hidden Rebel base on Yavin IV.

The schematics reveal a hidden weakness in the Death Star's thermal exhaust port, which could allow the Rebels to trigger a chain reaction in its main reactor with a precise proton torpedo strike. Han leaves the Rebels to pay off Jabba, after collecting his reward for rescuing Leia. Luke joins their X-wing starfighter squadron in a desperate attack against the approaching Death Star. In the ensuing battle, the Rebels suffer heavy losses as Vader leads a squadron of TIE fighters against them. Han and Chewbacca unexpectedly return to aid them in the Falcon, and knock Vader's ship off course before he can shoot Luke down. Guided by the voice of Obi-Wan's spirit, Luke uses the Force to aim his torpedoes into the exhaust port, destroying the Death Star moments before it fires on the Rebel base. In a triumphant ceremony at the base, Leia awards Luke and Han medals for their heroism.