Flash of Two Worlds

"Flash of Two Worlds" is the second episode of the second season of The Flash, and the twenty-fifth episode overall. It aired on October 13, 2015.

Synopsis
Jay Garrick, a mysterious man from Earth Two, appears at S.T.A.R. Labs with a dire warning about an evil speedster named Zoom, who is set on destroying The Flash. Barry and the team must decide if they can trust this stranger even as they face yet another powerful meta-human. Meanwhile, Joe must deal with a determined officer named Patty Spivot who wants to join his meta-human task force.

Trivia

 * The episode is named after the iconic story of the same name published in September 1961, featuring the first crossover between the Barry Allen and Jay Garrick iterations of The Flash.
 * Joe calls his colleague "Jimmy" to corroborate Eddie Slick's alibi that he was in Blackgate Penitentiary at the time of the particle accelerator explosion. Given that Blackgate is traditionally located in Gotham City, one would assume "Jimmy" is a reference to Commissioner James Gordon of the Batman mythos.
 * Professor Stein says that on Earth Two, Joe West's doppelgänger might be an Nobel Prize winning physicist (like his comic counterpart, Ira West, Iris' adoptive father). It was later revealed in "Welcome to Earth-2" that he was in fact a musician.
 * Cisco says there are 52 dimensional breaches, likely a reference to the 52 parallel Earths in DC Comics which are part of the New 52 continuity.
 * After the Sand Demon is defeated, Patty Spivot called for the Flash. Both Barry and Jay say "I'm coming" and run towards her with a brick pillar in the middle of them, re-creating the cover of the comic "Flash of Two Worlds".
 * When Cisco is talking to Dr. Martin Stein about his powers, he says "I get a little vibe". In the comics, Vibe is Cisco's meta-human alter ego.
 * The number on the side of the fire truck is 52. This is likely a reference to the New 52.