Barry Allen (Grant Gustin) will face a terrifying foe unlike anything he has ever encountered in the upcoming episode of "The Flash."

In "Flash Back," the synopsis reveals that the speedster is desperate to find a way to defeat the supervillain Zoom, who is actually his former friend Jay Garrick (Teddy Sears). He believes that he needs to increase his speed to get the upper hand in the fight and no one in the present time can help him accomplish that. Barry needs to travel back in time to find Eobard Thawne/Dr. Harrison Wells (Tom Cavanagh).

Barry's plan is to fool the scientist by pretending to be his earlier self. He thinks that the masquerade will be enough to get Thawne to teach him how to run faster. Unfortunately for Barry, he will have to undergo several hurdles to be able to see the other man. First, Barry should learn how to escape capture of a frightening specter that preys on those who meddle with time.

The promo shows the phantom ceaselessly pursuing Barry. The vision is enough to make anyone nervous. There is a scene where it almost catches up with the speedster, as Barry is seen cowering while the wraith prepares to descend on him. No one knows yet what happens to people after the specter is finished with them. They may have ended up dead or their souls destined to roam around forever.

Aside from the wraith, Barry will also have to face Hartley Rathaway (Andy Mientus). The exact time in the past which the speedster will travel to is around the time his old self was fighting with the meta-human. Eobard himself will get wise of Barry's Machiavellian plan. He and Rathaway will track down the fleeing speedster, hoping to kill him and the future's course.

"The Flash" season 2 episode 17 airs Tuesday, March 29 at 8 p.m. EDT on the CW.