Rogue one most of the action shortened. (Start of the movie till Galens death).
Coming off from what @Bevan was saying he did mine and research Kyber Crystals as they could contain a large amount of energy. If the DS-01 had only a few Kyber Crystals it wouldn't be able to destroy much so the Empire went and sought out large amounts of Kyber Crystals. The planet that we saw the Kyber Crystals being harvested from was Jedha a planet that Jyn Erso was familiar with due to her being taken there by Saw Gerrera a few years back since her mother died. This is when Jyn got rescued and got put on a mission by the Rebel Alliance to hopefully learn more about this weapon of mass destruction. The first part of Jyn's mission was to kidnap and take the defected imperial pilot who was on Jedha with Saw Gerrera. She got placed with a Rebel Captain called Cassian Andor and his imperial reprogramed droid "K-2SO". At first Jyn hated the two and then fell in love with Cassian at the end of Rogue One. So they got to Jedha and were stopped by an imperial convey (10-20 Storm Troopers and 2 Tank Troopers). Cassian noticed some movement in the crowd and realised some of Saw's goons were about to attack the convey for their compacted Kyber Crystal so they can sell it for a good amount of credits. Before this happened and before they saw the convey they met 2 force worshippers named Chirrut Imwe and (I forgot the other one). These two force worshippers saved Jyn and Cassian and then they all got captured because during the fight Cassian shot one of Saw's goons. The then got taken to Saw Gerrera's base of operations and got locked up except for Jyn because Saw Gerrera and Jyn know each other from the past. Saw Gerrera shows Jyn the message the imperial pilot had which was from her Galen Erso and she then starts to cry (It was a pure heart ranching moment). The DS-01 slowly comes into picture and right after all the imperials leave the city of Jedha they blast it with a small amount of LAZER and completely blow up the city and most of it's surrounds including Saw's base which Jyn, Cassian, two force worshippers, K-2SO and the imperial pilot escaped from. Saw Gerrera saw that he wouldn't make and died (A truly heroic death).
They then get back to the base and find Galen Erso's where about and decide to go there. Jyn thought they were going to rescue her father but before they took off one of Cassian's officials told him to not bring back Galen alive (Try and kill him for short). They make it to the planet and crash due to the environment and weather being so tough. Cassian then took the imperial pilot with an armed gun to go shot Galen Erso. One of the worshippers took notice of this and warned Jyn of what they're doing and she decides to go. Director Krennic Orson then arrived in his big shuttle. Jyn didn't know this because she was to busy climbing up a long ladle from the bottom of the cliff straight to the top of the platform where her father was situated. Director Krennic then had an argument with him and shot all the other scientist and left Galen, by this time Jyn was crawling over the side of the platform and Cassian called in a Rebel fighter attack/bombing run. One of these bombs hit the middle of the platform when Galen and Krennic were standing and Galen got the brunt of the explosion so he was left dying on the floor. Jyn killed a few Storm Troopers with her pistol then she rushed over towards her father who was dying. She starts crying and Galen says a couple of words then dies. She gets pissed at Cassian then they go on to gain the DS-01 plan and boom the film ends.
But yes @Snow and @Gregis He did research the Kyber Crystals and did work for the Empire not knowing that he was helping the building process of the DS-01. Sadly he didn't know until it was to late. Galen has a whole lot more jobs to do then just build the DS-01.