Fresh off his recent stint on CBS’ “Supergirl,” Barry Allen (Grant Gustin) returned to the CW in “The Flash” episode 17, titled “Flash Back.” With the hunt for the villain Zoom more personal and pressing than ever, the hero tries something a little desperate in order to increase his speed and become strong enough for the big fight. 

The episode opens with a very focused and very frustrated Barry trying to crack the equation that will make his speed increase. After discovering that Zoom is the man they thought was their friend, Jay Garrick (Teddy Sears), or at least someone who looks like him, he’s really motivated to finally stop him. However, things are not going well for him, and Caitlin Snow (Danielle Panabaker) even posits that he simply might not be physically capable of reaching the same speeds as Zoom. 

Later that night, Barry is at dinner with Joe (Jesse L. Martin), Iris (Candice Patton) and Wally (Keiynan Lonsdale) when he gets an idea. Hearing Wally mention that he’s learning a lot of things for his new project by reading his idol’s journals, Barry realizes he needs to consult an expert. The only other speedster that’s been faster than him was Eobard Thawne (Tom Cavanagh), the big bad guy of Season 1. It takes some convincing, but Cisco (Carlos Valdes) and Caitlin agree to let him use the speed force to travel back in time to ask the Reverse Flash for his advice. 

Cisco suggests that he go back to the day when Thawne, then posing as Harrison Wells, was distracted by the return of his protégé, Hartley Rathaway (Andy Mientus). Unfortunately, when the hero gets back to the past, he finds that he showed up a bit too early. He’s at the moment when he was just capturing Hartley. During a lull in the fight, future Barry pulls past Barry in an alley and, after a brief mirror match, injects him with something from Caitlin that will knock him out for enough time to complete his mission.

Just as he did a year ago, future Barry brings in Hartley, but this time he remembers that the villain used his ear implants as a bomb to escape captivity. He subtly cautions Cisco to take the implants from him, much to the surprise of Thawne, who rolls up in the wheelchair that only Barry knows he doesn’t need.

Without being discreet about it, Barry starts to grill Thawne for information about the speed equation. The villain is clearly getting wise to him when a call comes out from the Central City Police Department. A security guard was looking around Barry’s lab when some kind of wraith monster emerged and attacked the officers inside the station. The hero arrives just in time to see Eddie Thawne (Rick Cosnett), his friend who sacrificed himself at the end of Season 1 to stop his distant relative. Previously, Iris was lamenting that she had been missing her fiancé-to-be, so the moment was extra emotional for the speedster. 

When he gets back to S.T.A.R. Labs to help track the wraith, Thawne asks him to step away for a second. When he does, the villain knocks him out. The Flash wakes up inside Thawne’s private holding cell and tells him the jig is up. The monster that’s after him is known as a Time Wraith, and it tracks speedsters that have disrupted the natural order of things. Thawne knows how to avoid them, but Barry isn’t as experienced with time travel. 

With all their cards on the table, Barry tells Thawne that he’s got him at a stalemate. He lies and tells the villain that his evil plan works and that if he doesn’t help him, past Barry will wake up with a note explaining everything, ruining his upcoming victory. Backed into a corner, Thawne reluctantly agrees to help.

While the two enemies are having their tense, paradoxical conversation, the wraith finds its way to S.T.A.R. Labs and attacks Cisco, Caitlin and their new prisoner, Hartley. With the help of the criminal’s sonic gauntlets, they manage to fight off the wraith, who they keep comparing to a dementor from “Harry Potter.” Later, Cisco comments that Hartley isn’t such a bad guy after all.

Suddenly, when it looks like the plan to travel back in time is back on track, past Barry rushes in and demands to know what’s happening. Apparently, the injection wasn't as strong as future Caitlin thought it would be. At the risk of letting his questions further change the course of history, future Barry reveals his identity and explains that the Time Wraith is after him. Before he can explain more information from the future, such as Thawne’s identity, the villain shuts him up and explains that he’s dangerously close to destroying his timeline. He takes Barry aside and hands him a flash drive with all the information he needs to crack the speed equation. Although the hero isn’t too keen to take Thawne at his word, he’s got no other choice. 

The Wraith is on its way back to S.T.A.R. Labs, so there’s very little time to form a plan. Barry trusts his friends and immediately returns to the future, with the understanding that they will use the following year to figure out a way to stop the Time Wraith. Sadly, when he gets back, Cisco’s device does nothing but annoy it. Just when it looks like it’s about to kill Barry, Hartley enters and subdues it once more with his sonic gloves. The hero stands up expecting a fight, but it turns out that his meddling in the past somehow led to Hartley’s rehabilitation in the following year. Ciaco and Caitlin seem to trust him and he’s harmless as a butterfly joking that he’s got a very important dinner date with his parents. 

They take the drive that Thawne gave Barry and find that it’s filled with loads of complicated-looking math equations that will no doubt help Barry achieve a higher speed. The episode ends on a somber note as the speedster returns home to Iris, who is looking through old photo albums of Eddie. He tells her that she needs to move on, but has something for her that will help. He takes out his phone and reveals that he got Eddie to record a video message to her while he was in the past. 

With his speed reaching new heights and his meddling in the past complete, the road is clear for a confrontation with Zoom.