During the Galactic Civil War against the Galactic Empire, two Rebel pilots in T-65B X-wing starfighters were patrolling near Scarif when they discovered the Galactic Empire building the Death Star.
In 21 BBY, the Galactic Republic began building the Death Star in orbit over the planet Geonosis. In 9 BBY, it moved to another planet and orbit and prepared for the last piece.
Mon Mothma sent the two scout pilots to find out what the Empire was doing. They jumped out early but then, they jumped and came out again when they saw the fleet of Star Destroyers and the Death Star and caught by the TIE fighters. Scout Three had been hit, Scout Two then told his droid friend R4 to transmit the scans to Base One in Yavin 4 what they had discovered the space battle station the ultimate weapon. But he's stuck and hit by one of the Star Destroyers.
The rebels forces failed to transmit the scans to their base. Mothma and General Davits Draven sent for another mission to get Jyn Erso from the Imperial prison in Wobani and have her taken to their headquarters. She was informed of her father's involvement in constructing a superweapon and given the option to either help the Alliance find Saw Gerrera on Jedha or be thrown back into prison. In exchange for helping, she would receive a clean slate from the Alliance. Jyn agreed and met with Gerrera on Jedha, where she found a recording from her father that revealed he had built a fatal flaw into the Death Star, the Empire's planet-killing battle station. The only way to discover the flaw would be to steal the Death Star plans from the data vault in the Citadel Tower on Scarif.
Believing itself outmatched against the Empire and its battle station, the Alliance initially refused to send its forces to Scarif. In defiance of orders, Jyn and Andor led a team called Rogue One to steal the plans from Scarif. Upon learning of their actions, the Alliance High Command sent its forces to join the Battle of Scarif. Rogue One successfully stole the plans and transmitted them to the Alliance Fleet, but at the cost of their entire team once the Death Star fired on the planet. The Alliance analyzed the plans and used them to destroy the Death Star during the Battle of Yavin, just as they had hoped to do when they freed Jyn from custody.
The discovery of the Death Star is the focus of Rogue One: Recon A Star Wars 360 Experience, a 360 experience that was developed by ILMxLAB and distributed by the telephone company Verizon.
- Star Wars: Timelines