Versus Zoom

"You can't lock up the darkness."

- Zoom taunting Barry

"Versus Zoom" is the eighteenth episode of the second season of The Flash, and the forty-first episode overall. It aired on April 19, 2016.

Synopsis
Equipped with the tachyon device, Barry believes he is fast enough to stop Zoom and wants to open the breaches to catch him. Harry strongly advises against that plan but after Barry convinces him, Harry reveals that Cisco has the power to re-open the breach. Meanwhile, Hunter Zolomon's painful story on Earth-2 and his origin as Zoom are revealed.

Trivia

 * While Barry tests out the tachyon device, he travels through a breach, coming out through a breach. Once he comes back to S.T.A.R. Labs, he asks "How long was I gone?", but neither Cisco nor Caitlin pay attention to the statement. This is a reference to, an episode of Supergirl in which The Flash crossed over. This episode took place within this moment.
 * Strangely, when Wells tells Barry that there is no way back to another Earth, Barry doesn't tell him about how he had accidentally run to another Earth, which had happened earlier that episode (most likely due to his uncertainty about how that happened and his inability to precisely create such a breach).