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Warning: This post contains spoilers from The Flash’s 100th episode, which aired Tuesday night. Read at your own risk!

A familiar character made a surprising return in The Flash’s 100th episode.

In the milestone hour “What’s Past is Prologue,” Barry (Grant Gustin) and his daughter Nora (Jessica Parker Kennedy) traveled back through the show’s history to collect items they needed to defeat Cicada (Chris Klein), season 5’s big bad (or so we thought!). During their journey to the past, Nora and Barry encountered Eobard Thawne, a.k.a the Reverse Flash (Tom Cavanagh), which lead to Nora finding out that Thawne murdered her grandmother. After returning to the present (and Team Flash’s subsequent failure to capture Cicada), Nora sped off to 2049 to see Thawne, who is imprisoned in Iron Heights and still sporting Wells’ face after “Crisis on Earth-X.”

Yes, this means you think it does: Barry’s greatest foe is back (he promised he would be) and has been working with Nora all along. The evil speedster’s return in the final scene of the 100th episode starts to answer questions that both the audience and Sherloque Wells (Cavanagh) have had since the beginning of the season: Why did she finally decide to interfere in the season 4 finale? Who is she working with? And how did she learn to use her speed after she removed the power dampener her mother implanted in her? According to executive producer/showrunner Todd Helbing, viewers will learn more about Nora and Thawne’s relationship when the CW superhero drama returns in the new year.

Below, EW chats with Helbing about the 100th episode’s big twist, what Team Flash’s failure to defeat Cicada means, and what else is in store for Sherloque.

ENTERTAINMENT WEEKLY: First, is this the version of Thawne that escaped at the end of “Crisis on Earth-X”?

TODD HELBING: Yeah. So, he escaped from “Crisis” and then, at some point, went to the future, and got put into Iron Heights.

Is this something you’ve been planning since Nora was first introduced in “Crisis” last year?

We wanted to do the Nora thing all last year, obviously, and then have her be a big part of season 5. We had, actually, a wildly different version of season 5 at the midpoint of last season. I think after we watched the crossover (personally, Thawne is my favorite villain), everybody in the room started talking about seeing him again and how great it was and the reaction from the fans. We just thought we should figure out a way that we could weave him into season 5, and once we start talking to him, the dam broke open and we figured out a pretty cool way to do it.

What should the audience take away from this final scene — that Thawne is the one that sent Nora back to the present?

Some of the stuff I want everybody to get in the moment, but I would say that he has been instructing her in different ways. You’ll really get to see the relationship moving forward in the back half of the season.

How would you describe their relationship?

I would describe it as he’s her coach.

One of the questions we’ve had all season is how Nora learned to use her speed. Is this scene supposed to answer that question?

In episode 10, we give the audience the information as to how they’re working together.

At the beginning of the season, I saw a theory that Thawne was behind Nora’s arrival. Were you aware that people started guessing that so early on, and if so, did that surprise you?

No, I think the audience is pretty smart. In the first half of the season, there’s a lot of little Thawne references — even in the premiere when Barry is recounting the first Speed Force speech that he got from Thawne, and we flash to him. I wasn’t really surprised, but I was surprised by one section about what I read. You know, it’s one of those things that just because the people in the audience aren’t in the writers’ room, our version is always different and it’s just depending on the degrees of difference that they get. I think there are parts of it that people cue into every season, but it’s kind of impossible for somebody to nail it exactly right.

Was Cicada part of Thawne’s plan along? There’s also that moment of recognition when Nora mentions Cicada when she and Barry return to the “Flash Back” episode.

Let me see how much of this I can answer. Cicada always existed, but like we explained, not that version of Cicada. So the reason for that is going to play into the back half of the season.

Team Flash failed to defeat Cicada in the episode. How does this latest confrontation affect Cicada’s plan for the rest of the season?

Now he’s on the hunt and they know who he is. CCPD is brought into the fold, so he’s a wanted man. I’ll just say that this is sort of our act two. As you start to understand more of the Thawne relationship with Nora, you’re going to see how everything starts to play out with Cicada. He’s really under the gun here, so his time to do what he wants to do is running out.

Does that pressure make Cicada more desperate or does he start acting more strategically?

In the first eight episodes, Team Flash isn’t necessarily his number one objective; it’s just metas in general. They’re sort of now thwarting his plan, so I think his ire has started to shift a little bit. He’s more concentrated on stopping Team Flash than he was initially.

Image zoom Jack Rowand/The CW

Sherloque is getting closer to figuring out what’s going on with Nora. Can you tease what we can expect from that part of the story?

From the first episode that he appeared, he knew that something was wrong with her story and, just being a detective, he looks at everything as a mystery. He’s starting to slowly put these pieces together. He’s going to run into a couple roadblocks. One episode that I’m really excited about is you get to see one of the wives comes back, which is really fun. Everything is a mystery to him, including love, so he’s trying to solve all this, but he starts to slowly decipher what it is about Nora’s story that doesn’t make any sense and how these pieces are starting to fit together, and then it’s all going to come to a head.

What can we expect from one of his ex-wives returning to his life?

It’s just the fact that he married five women seven times. You get to learn why, you get to learn who these women are, you get to learn why every marriage fails miserably, and you get to see this little mystery that he’s been unable to solve his whole life. You sort of start to feel for the guy.

How does this journey to the past impact Nora’s relationship with her parents going forward?

The great thing about sci-fi is you always get to tell familiar stories but with a twist. Having your child return and get to know you as an adult and not experience parenthood in the same way as most people, it was a lot of fun for us. As a parent, you don’t necessarily want your kids to know everything about you when you were younger, or what you went through when you were starting out, especially not when you’re a speedster and you’ve gone through some of these things that Barry has gone through. I think your natural inclination is to protect your children from some of this stuff. So, exposing Nora to the fact that Savitar was really this bad version of yourself and — Barry obviously doesn’t know that she knows [Thawne] already — [introducing] her to this evil person that’s been such a big influence in his life, there’s no way that she can’t get a completely different viewpoint of her dad after that experience. Ultimately, it’s going to bring them together. You know, that scene outside of the Allen house sort of sums it all up for me — the look on Nora’s face when she sees Barry watching [because] he knows what’s going to happen. The way that Jessica played that, I think, was just perfect. But yeah, I think she just has a really complete picture of who her father is.

Was there anyone you wanted to bring back for the 100th that you weren’t able to?

Originally, we had Eddie Thawne [Rick Cosnett] in it. It’s hard with episodes like this when you have 42 minutes to tell the story to get everybody back. There are scheduling conflicts. I think we would’ve loved to have a longer scene with [Teddy Sears] for sure. We would’ve liked to get Tom Felton back, that would’ve been great. There were a lot of people that we wanted that were on the board that you ultimately can’t do just because of time. Patty Spivot [Shantel VanSanten] would’ve been great.

The Flash will air on a special night — Sunday, Dec. 9, at 8 p.m. — as part of this year’s Arrowverse crossover “Elseworlds.”

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