Airing the day after the Supergirl / The Flash crossover episode, season two’s episode 17 of The Flash found Barry traveling back in time in order to learn the secret to increasing his speed from the one person who knows the answer: his arch-nemesis Dr. Harrison Wells/Eobard Thawne (Tom Cavanagh). Barry’s fully aware going back in time could have a major impact on the timeline, but he has to take that chance as he’s run out of options. If Barry can’t increase his speed, he’ll never be able to take down Zoom. Episode 17 also found Andy Mientus reprising his role as Pied Piper/Hartley Rathaway as well as a heartbreaking reappearance by Rick Cosnett as Eddie Thawne.
Barry’s pissed at himself for trusting Jay Garrick who’s been revealed to be Zoom when he should have learned his lesson about trusting people from Earth-1’s Dr. Harrison Wells. Barry thought both men were his mentors but in truth both just wanted to use him. He’s now determined to train harder and run faster than anyone in history so he can stop Zoom. In comes Cisco (Carlos Valdes), walking past Jay’s helmet and asking if they can put it away because it gives him daymares. Barry (Grant Gustin) won’t let him put it away because he wants to be constantly reminded to stay on task. Cisco’s got books on speed, and Caitlin (Danielle Panabaker) figures out that Reverse Flash, Zoom and The Flash all move their legs at the same speed, but Zoom and Reverse Flash’s feet are on the ground for less time. Caitlin asks Barry if maybe he can’t run faster because he’s simply incapable of doing so physically.
At the West house, Barry, Iris (Candice Patton), and Joe (Jesse L. Martin) are sharing wine and commiserating over not seeing the Jay/Zoom thing coming. Iris tells Barry she went on a date with her boss and that he’s sweet, smart, and he makes her think about Eddie. She confesses that she thinks about him every day, and Barry says Eddie would want her to find love again. She’ll know when she’s ready, says Barry. Wally pops in, Joe serves up the food, and dinner conversation returns to normal. Wally’s explaining more about a project he’s working on about speed engines and he read Ferrari and Porsche’s journals to figure out how to get faster. That gives Barry an idea and he rushes out.
Barry wants Eobard Thawne/Harrison Wells to teach him how to run faster by going back in time, but Cisco and Caitlin think that’s crazy as does Earth-2’s Harrison Wells. Harrison’s preoccupied and upset that he hasn’t found Jesse (Violett Beane) and he says going back to Thawne is a horrible mistake because the timeline will be altered and if anything happens, only Barry will know it. Still, Barry’s convinced he has to do it or he’ll never beat Zoom making the argument that when he went back to see his mom die, he didn’t affect the timeline. Of course Cisco and Caitlin ultimately get on board and plot out when he should go back and how he should do it. Cisco doesn’t want him to set off a 12 Monkeys time loop he’ll never get out of. They hug it out and Barry’s gone. Racing through time he lands in history but he’s arrived too early.
The Flash from now, referred to from here on as Flash-1, sees The Flash from back then, or Flash-2, on the street dealing with Pied Piper/Hartley Rathaway (Andy Mientus) who’s disabling Team Flash’s comms. Flash-1 zooms in, grabs Flash-2, and knocks him out with a shot Caitlin prepared. Flash-1 switches the emblem on his chest with the emblem on Flash-2’s outfit and heads off to take care of Hartley, capturing him and transporting him back to S.T.A.R. Labs. They lock him up and The Flash knows the things in Hartley’s ears are bombs, warning Cisco and Caitlin about the ear plugs so Hartley can’t break out. Wells comes in and Barry starts peppering him with speed questions. Wells wants to know what inspired Barry to take this unorthodox approach and Barry says he wants more speed to take down the man in yellow. Wells agrees to help.
Joe and Eddie are talking about Wells downstairs at the police station when some creepy looking dementor beastie thing shows up to scare the pants off everyone. Wells sends Barry out to take care of it, and then stares at Barry’s equation on the board after he leaves. Barry arrives at the station and sees Eddie alive and is momentarily stunned.
Joe’s looking through all the damage and Barry’s still having a hard time processing the fact he’s talking to a living, breathing Eddie. Eddie takes off to check in with Iris and Joe thinks Barry’s being weird to Eddie because he’s still into Iris, but of course that’s not it at all.
Back at S.T.A.R. Labs, Cisco wants to find the dementor that destroyed parts of the police headquarters. Barry slips up and says he’s seen the dementor before but sort of covers his mistake saying it was earlier that same day. Wells takes Barry into his office to work on the speed equation, but knocks him out and chains him up because he’s figured out Barry’s not Barry. Wells knows the dementor is actually a time wraith who’s after someone who’s traveling through time and doesn’t know what they’re doing. Harrison stands up and Barry doesn’t even flinch, confirming Barry is not the Barry from this time period. Harrison knows he’s from the future, and Barry calls him “Thawne” and tells him to let him go. Thawne won’t let him up until he says why he’s there, and Barry says it’s because Thawne’s the only one who’s figured out the speed force, asking how he manipulated it. Thawne’s trying to reason it out and quickly figures out he hasn’t beaten Barry because Barry’s alive in the future. That means his plan has failed. Barry explains that’s not true and that Thawne won. He goes on to explain a singularity had formed and he needs Thawne’s help to get back. But now Thawne figures out he doesn’t need Barry at all and that he should have thought of that before he came back. “You ran all the way back here just to die,” says Thawne. But Barry says there’s a hidden letter that tells this Barry everything and Thawne will never make it back home if he kills him. Barry has the advantage and warns Thawne he has to help him or else.
At S.T.A.R. Labs, Cisco meets with Hartley to find out how his gauntlets work. As Caitlin works at her computer, the time wraith shows up and chases her. Cisco and Caitlin lock themselves in with Hartley as the time wraith attacks. Barry tells Wells they have to help them, but Hartley’s hoping he can use his gauntlets to drive the time wraith away. The gauntlets work and the time wraith flees, with Harrison (now back in his wheelchair) and Barry showing up after the wraith’s gone. Cisco and Caitlin need to figure out how to fight the time wraith and take off to work on Hartley’s gauntlet. When they leave Harrison says time wraith’s hate when speedsters travel through time and so speedsters usually do their best to avoid them.
At the police station Barry looks for clues as to what the time wraith is so they’ll know how to get rid of it. Eddie comes in and Barry asks him if he’ll do a video message for Iris for her birthday even though it’s two months away. He tells Eddie to speak from the heart and talk as if “it was the last thing you could ever say to her.”
Back at the lab, Cisco says he doesn’t know how to manipulate the frequency and needs Hartley’s help. Wells okays it but only if Hartley stays in the cell. Just then Flash-2 shows up demanding to know where the other Flash is. Cisco and Caitlin are just as confused as Flash-2 and Flash-1 tries to introduce himself, but Cisco is so completely confused and Flash-1’s not making things any easier. Wells butts in to say Barry is from the future which makes Flash-2 super excited because that means he can time travel.
Wells gives Flash-1 the key to running faster: tachyon enhancements. Handing Barry a flash (how appropriate!) drive, Wells says everything he needs is on it. Barry will be able to use it in his time to get fast and Wells all but pushes him out the door saying, “Time to go home, Flash.” Before he leaves, Barry whispers to Cisco that Hartley knows where Ronnie is. He switches back his Flash emblem with Flash-2, thanks them all, and takes off with the time wraith hot on his tail. He building up speed racing around S.T.A.R. Labs and it’s not working until Flash-2 shows up to keep the time wraith from closing in on him. Together the time wraith and Barry wind up in Barry’s time, and Cisco can’t stop it but Hartley can with his gauntlets. Cisco, Caitlin, and Hartley are relieved but Barry’s now the one who’s confused. Hartley says it’s been a pleasure working with them and takes off to have dinner with his parents. Barry, Cisco, and Caitlin put in the drive but nothing happens until Barry pounds the podium in frustration. That opens up the drive and Barry says, “We’ve got it!”
Now back in his own time, Barry goes to check on Harrison who’s depressed and disappointed he can’t find Jesse. Barry tells him he did what he could to protect his daughter, but Wells continue to blame himself for losing her. All he ever wanted was her love.
Barry goes home to find Iris looking through photos of her and Eddie. It’s been a year since they lost Eddie, and Barry asks when she will move forward. She doesn’t know and Barry shows her the video he took of Eddie back in time, saying he was putting together a video montage last year for her birthday and didn’t give it to her then. Eddie’s emotional message has Iris in tears, with Eddie calling her fierce, independent, loving and kind – and the best decision he ever made. He tells her she deserves to be happy.
In a voice-over Barry says you have to deal with things from your past before you can move on. He picks up Jay’s helmet and says, “I’m coming for you.”