During the Clone Wars, Galen and his family were abducted by Separatist agents but were rescued by his old friend Orson Krennic. Erso worked on manipulating Kyber crystals and was encouraged to continue his studies by Krennic.
Prior to the rising of the Galactic Empire, the Rebel Alliance intercepted a coded transmission from Galen indicating that a major weapons test was imminent, resulting in the Rebels recruiting his estranged daughter to partake in a mission to steal the plans to what they would discover to be the Imperial superweapon known as the Death Star.
He would die in a firefight between the Imperials and the Rebellion, but not before saying goodbye to Jyn.
Role in the Film
Galen Erso first appears and is shown to be living a humble farming life with his family on Grange. He tells his daughter Jyn to escape with his wife Lyra as Orson Krennic arrives to take him back to continue work on the Death Star. He expresses a distrust in the Empire, lying to Krennic that his wife is dead. After the Death Troopers search his home, his wife reappears and shoots Krennic and is killed, Galen disappears and is taken back to the Empire.
After Jyn Erso and the rebels are taken to Saw Gerrera's base, his message about a flaw in the Death Star is shown to Jyn by Saw before the planet is destroyed.
Galen is later shown to be on the Imperial refinery of Eadu, where he and the rest of the Imperial scientists are lined up by Krennic to reveal the one who had sent a defector to the Rebel Alliance. Galen reveals himself after Krennic threatens to kill all of them, but the Death Troopers kill all of them anyways. Galen is later killed by a torpedo blast by incoming Rebel fighters, though he manages to see his daughter before he dies.