Around 2 BBY, the Rebel Alliance was officially declared, with the Galactic Empire unwittingly unleashing it on the galaxy. At least one year later, Alderaanian Captain and secret Rebel Alliance member Raymus Antilles arranged to ship C-3PO and R2-D2 to the Star Tours intergalactic travel agency to ensure the successful expansion of its operations. This move led the Galactic Empire to suspect that the travel agency was a Rebel front.
Imperial starships raid the station
Sometime later, around 0700 hours one morning, a Rebel spy boarded the StarSpeeder 1000 for Star Tours Flight 1401 (ST-1401) among various civilian passengers. Due to a last-minute mix-up, protocol droid C-3PO replaced AC-38 as the pilot. Learning of the spy's location, the Galactic Empire dispatched a fleet composed of several Imperial-class Star Destroyers and several TIE fighters to surround Spaceport THX1138. Imperial agents including a contingent of stormtroopers, under Darth Vader's orders, boarded the station and identified one of the passengers as the Rebel spy just before the speeder's departure. When they attempted to stop the ship and board it, astromech R2-D2 opened fire with the ship's blasters and took off, firing on Imperial ships before escaping into hyperspace with the Rebel spy. Prior to its escape, however, Vader had hired the Mandalorian bounty hunter Boba Fett to retrieve the vessel in case it escaped.
This article is based on the beginning of the Star Tours: The Adventures Continue ride. The ride randomly chooses one of two conflicting sequences of events.
One possibility is that ST-1401 is cleared for takeoff, only for several dozen stormtroopers, four skytroopers, four TIE/LN starfighters, and Darth Vader (riding on a Hover Platform) to descend on the runway and force ST-1401 to stop. After Vader reveals that a Rebel spy is aboard and declares the spy's arrest, ST-1401 attempts to escape by backing out. However, Vader stops it and attempts to reel it in via the Force. R2-D2, however, activates the StarSpeeder 1000's weapons systems and fires them at Vader. Vader is forced to release the vessel from his grip in order to deflect the cannon blasts with his lightsaber, with the stormtroopers proceeding to fire at the escaping vessel and the TIEs taking off in pursuit. R2-D2 then destroys one of the TIEs upon escape, forcing the other three to scatter, before taking off into hyperspace.
Another possibility is that ST-1401 is scanned by an Imperial Prowler 1000 exploration droid before it can clear departure regulations. In addition, then-smuggler Han Solo, whose ship, the Millennium Falcon, is docked next to the StarSpeeder 1000, has a run-in with stormtroopers that were presumably deployed to capture ST-1401. Afterwards, both the Millennium Falcon and ST-1401 escape the station, with ST-1401 on the Falcons tail while evading Imperial ships, and the Falcon aiding ST-1401 in escaping. The Falcon eventually attacks one of the Star Destroyers, severely damaging it, thus allowing the StarSpeeder 1000 to escape from the Imperials into hyperspace.
Although the StarWars.com blog entry "Convenient Daily Departures: The History of Star Tours" confirmed Vader's involvement in the raid, it did not specify whether Vader had personally led the troops in attempting to halt ST-1401. As such, the article is written with events from both scenarios.
When sequel trilogy content was added to the attraction in 2017 to promote the release of The Last Jedi and allow an earlier The Force Awakens update to exist separately from the original 2011 material, reshot versions of the raid scenes featuring Kylo Ren and First Order stormtroopers in place of Imperial counterparts were added. Other than these visual and audio changes, the events are basically identical.