At the age of 11, Allen's (Grant Gustin) mother died unexpectedly, and his father was regarded as the murderer. Afterwards, detective Joe (Jesse L. Martin) took in the lonely Allen. Under Joe's influence, Allen became a CSI forensic analyst as an adult. In the process, he never gave up the investigation of the real cause of his mother's death. Allen was very fascinated by the particle accelerator studied by scientist Harrison (Tom Cavanagh). However, an accident caused the accelerator to explode, and Allen was hit and fell into a coma. Nine months later, Allen, who finally woke up, was surprised to find that magical changes had occurred in his body, and therefore gained special abilities. As time went on, Allen began to understand that there were many people in the world who had superpowers like himself, and protecting the safety of citizens became his new responsibility and mission.