 In the process, he was drawn into the Speed Force (the first explicit use of the concept in the DCAU), and barely managed to escape.The episode "Flash and Substance" is centered on the opening of the Flash Museum on "Flash Appreciation Day" in Central City, and featured many of the Flash's rogues in cameos, while focusing on Captain Boomerang, Mirror Master, Captain Cold, and The Trickster (voiced by Mark Hamill).Linda Park also appears as a reporter covering the museum opening.