Learn how police scientist Barry Allen, the Flash of the 1960s, first crossed paths with Jay Garrick, the Flash of the 1940s, in this hardcover collecting The Flash #123, #129, #137, #151, #170 and #173. These are the stories that first established the science fiction concept of parallel universes in DC Comics, as the Fastest Man Alive learned to use his super-speed to travel across dimensions to Earth-2!