The Flash must travel a year back in time in order to seek out the help of Eobard Thawne, who at the time was pretending to be Harrison Wells, unbeknownst to the Flash team at STAR Labs. Barry Allen needs to get faster to achieve his eventual goal of defeating Zoom, but will Thawne help him or kill him?

　　Warning: Spoilers for The Flash #217, "Flash Back," are ahead.

　　

5. The Flash has to Travel Back in Time, on Purpose

In the present, Allen has plateaued with his speed, and he thinks the only person that can help him is someone who truly understands the Speed Force: Eobard Thawne. Unfortunately, Thawne is dead, so Allen has to travel back in time to seek his help. Allen has travelled back in time before, but he's still new to the whole idea of using his speed to manipulate himself through time.

　　

4. A New, Deadly Force Appears

While travelling back in time, something passes Barry. Fans of The Flash comic book series may think it's the Black Flash, who is a symbol of death for speedsters. Thawne eventually explains what these creatures are and their purpose: "[a] Time Wraith follows someone who travels through time and doesn't know what they're doing."

　　

3. Thawne Confronts Allen

Allen tries his best not to give away he's from the future, by getting his past self out of the way and not letting his companions know what's coming down the line. However, Thawne is extremely intelligent and knows something is wrong, so he knocks Allen out and handcuffs him. Thawne eventually gets the information he needs from Allen and is about to kill him; however. Allen says there's a hidden letter which he gave to his past self and if Thawne kills him, Thawne's ultimate plan in that future will never happen. From here, Thawne is game to help Barry.

　　

2. There are Other Speedsters

From the inception of this series, viewers have seen numerous characters that have tapped into the Speed Force: The Flash, Reverse Flash, Jay Garrick, Zoom, and even the woman from episode 16 that used the Velocity 9 serum to gain super-powers. While Thawne and Allen are talking, he mentions in passing that there are other speedsters, aside from Allen and himself. Who are these other metahumans? We can assume one of the speed force users that Thawne is speaking of is Wally West, since he's already been introduced on the show. However, does that mean Thawne knows about Zoom or Jay Garrick or are there even more super-fast humans we have yet to learn about?

　　

1. Pied Piper is on the STAR Labs Team

One of Barry's other goals during his mission to get faster was to not alter time Obviously, Barry's actions changed the past, for better or worse. The Flash team enlisted Pied Piper's help to stop the Time Wraith. Piper was one of Flash's Rogues and ended up getting locked away at STAR Labs. However, in this new timeline, Piper is the only way that the team can get rid of the Time Wraith. Flash gets the Time Wraith to follow him to the present, and in that time, Piper developed a weapon to destroy the creature. After all is said and done, Piper is now an acquaintance to the team and no longer a criminal.

　　

Extra Development: Zoom is Hunter Zolomon

During the previews for next week's episode, it's revealed that Hunter Zolomon is Zoom, which could mean that the Jay Garrick they know, who may be dead, isn't Zoom. Earth-1 Jay Garrick is named Hunter Zolomon, which was revealed much earlier in the season. This doesn't answer everything, but it does give a bit more clarity to the story and a stronger connection to the comic book.

　　When Barry Allen messes with time, everything changes. Aside from Pied Piper now being a good guy, there are some other major things to think about. Eobard Thawne was supposedly defeated again, but did it go all the same way the audience saw in season one or just the way Thawne wanted everyone to see? Is he still out there? Allen doesn't know because he wasn't actually there to experience it.

　　The episode brings a few concepts from the comics books into the show. Aside from the idea that there are more than the few speeders the audience can currently see, the Time Wraiths, which resemble Black Flash and the current look of the Black Racer, enter the episode. Allen wants to get faster to be able to defeat Zoom, but as he does this, he keeps digging a deeper and deeper hole, creating problems all around him. Now, Allen has these creatures after him, and his world just got a whole lot bigger. It's getting to the point where he won't be able to do this all on his own.

　　Mat Elfring on Google+