As we saw in the trailer, Barry Allen (Grant Gustin) will have to face the villainous Danton Black, a.k.a. Multiplex (Micheal Christopher Smith), and this scene explains that Danton may be one of those exposed to dark matter when the particle accelerator exploded. Regardless of Multiplex's origins, we also see Barry having some second thoughts about using his gift of super-speed to become a crime-fighting vigilante.
While he is not shown in this clip, Caitlin Snow (Danielle Panabaker) also refers to Simon Stagg, who will be played by guest star William Sadler in this episode. We learn that Stagg actually stole all of Danton Black's cloning research and then fired him. Could Stagg be another target for the vengeful Multiplex? We'll have to wait and see, but in the meantime, take a look at this scene below, which also features Harrison Wells (Tom Cavanagh) and Cisco Ramon (Carlos Valdes).