Spoiler warning: Do not read on unless you’ve seen “The Flash” episode 204, titled “The Fury of Firestorm.”

While much of this week’s “Flash” focused on the team’s hunt for a new Firestorm, the biggest bombshells of the hour came courtesy of Iris’ (Candice Patton) long-overdue meeting with her mother, Francine (Vanessa Williams), and Barry’s (Grant Gustin) unexpected run-in with DC Comics villain King Shark, closely followed by an even more unexpected encounter with the Earth-2 iteration of Harrison Wells (Tom Cavanagh).

Executive producer Andrew Kreisberg and star Danielle Panabaker shed some light on the episode’s many revelations and what’s coming up in the impending “Arrow” and “Flash” crossover, which is designed to set up midseason spinoff “DC’s Legends of Tomorrow.”

Where’s Wally? Iris’ investigative chops led her to uncover that her mother secretly had another child eight months after Francine left Central City — a son with a name that should be familiar to DC Comics fans: Wally West (Keiynan Lonsdale). In comics canon, Wally is Iris’ nephew, not brother, but Kreisberg admitted that they dismissed the possibility of contorting Iris’ family tree to fit the comics fairly early on.

“We have these ideas in the previous season so we always knew we were leading up to this, and rather than them suddenly having some cousin that inexplicably… We always hated on TV shows that it’s year two and somebody’s like, ‘Well, Cousin John’s coming!’ And it’s like, ‘Oh, good ol’ Cousin John!’ who no one ever mentioned before. It was always weird. The notion that they don’t know Wally was where that came from, and then that was the idea that Francine was still alive and then that whole storyline,” Kreisberg explained. “Iris is now in the position that Barry was in last year – she’s keeping a secret to protect somebody and she’s going to find that, for all of her anger at Barry and Joe from last year, keeping this secret is not going to be so easy and it’s going to be weighing on her before she finally decides to take some action in an upcoming episode.”

And yes, conspiracy theorists, Kreisberg also confirmed that Wally is definitely “Joe’s son.”

Mama Drama: The downside of Iris’ journalistic digging was that she confirmed that her mother really is dying of a disease known as MacGregor’s Syndrome, which DC fans may recognize as a fictional disease that also afflicted Nora Fries, the wife of Batman villain Mr. Freeze in “Batman and Robin,” before being co-opted by “Arrow” as a terminal illness suffered by the Clock King, William Tockman, in season two.

“For all the reality of these shows — and part of the success of ‘The Flash’ and ‘Arrow’ is we try to ground them as much as possible — hearing someone say ‘I have cancer’ is a bummer and a half, and especially when you’re dealing with somebody who was also a drug addict,” Kreisberg explained. “You try to tread lightly on those things, because when you really start to analyze those things, especially episode three [“Family of Rogues”], it’s really an episode about violence and violence against children and parental abuse. It can be heady stuff if you really take a step back from it… For that [MacGregor’s] reference, for example, it’s a useful tool to say something like that — that sounds spooky and scary, without saying [cancer]. We used it on ‘Arrow’ and we all remembered it.”

The Devil You Know: The episode ended with a gobsmacked Barry staring down a familiar (yet different) face: the Harrison Wells of Earth-2. But next week’s episode won’t pick up exactly where we ended, according to Kreisberg: “The next episode opens in a slightly surprising way … I’m a fan of ‘Doctor Who.’ I think one of the things that Steven Moffat always does so brilliantly is that when he has cliffhangers and two-parters, they don’t just pick up exactly where they left off. You come in with an expectation and ‘oh wait, now I’m not quite where I thought I was going to be.’ Obviously, this scene will play out. but how it unfolds in 5, I think the beginning of 5 is really exciting. You’re going to get a lot of answers to questions you have.”

As for whether we’re seeing a Harrison Wells doppelganger or an Eobard Thawne-as-Harrison Wells doppelganger, Kreisberg would only tease, “I think part of watching the show is learning all of that stuff.”

Seeing Double: Kreisberg says “it’s about to go doppelganger-a-go-go on the show,” so we can expect more alternate universe shenanigans in upcoming episodes, and not just with Wells. “With time travel last year, we kind of tried to ease everybody into it and we tried to do the same thing here where the first episode was how the two guys look exactly alike and then Jay comes over and we establish the idea of Earth-2, and then we’ve had it playing in the background for episodes three and four just to remind everybody that the show is still ‘The Flash’ and you’re still going to get the typical ‘Flash’ episodes that tie into the normal mythology of the show. And now we’re really going to pick up, with 5 and with 6 and 7, the Earth-2 storyline. There will be doppelgangers and doubles.”

Although the producers were initially cautious of overwhelming the audience with too much multiverse action, Kreisberg said they came to realize “people have already watched a year of ‘Flash.’ Even the characters on the show, when someone flies in front of them, they’re not like, ‘What’s happening?!’ They’re like, ‘Oh, that’s probably a metahuman.’ Since the characters were more accepting of it, we realized the audience could be more accepting of it… That was why, when we first wrote episode 2, we wanted to see Jay fighting Zoom. That whole opening with Jay and Zoom fighting and seeing Earth-2, that was actually after the original conception. We realized people could handle it and see it and not be like, ‘What the hell is going on,’ like the show just suddenly turned into a David Lynch thing.”

Snow Day: The episode also saw a more assertive Caitlin, something that Panabaker said is set to continue this season, now that there’s a power vacuum where Wells’ leadership once was. “The old dynamic from season one was Wells — he created S.T.A.R. Labs, he was the boss and we all followed the law. And this year it’s feeling different. We’re trying to feel it out and suss it out and whoever has the most expertise or passion or chutzpah tends to be the one we follow.”

Kreisberg agreed, “It’s a conscious effort on our parts too. The thing that Iris adds to those scenes when she’s in the cortex is the heart that sometimes they don’t always have, being a bunch of scientists talking about things. Caitlin especially has taken the reins in a lot of ways, especially since they can all sit around arguing about the best thing to do. The other day we were watching a cut of an episode and it’s Caitlin saying, ‘No no, this is what we’re doing.'”

The episode also provided a measure of closure for Caitlin following the loss of Ronnie, which makes sense, given that she’s been growing closer to Jay Garrick (Teddy Sears). Kreisberg confirmed, “we’re building a relationship between Jay and Caitlin and there’s going to be some fun and surprises there,” but Panabaker also thinks she’s a little further along in her grieving process now.

“This grief for Caitlin is very different than last year. She was in a much darker, much unhappier place and I think there is a new purpose for her being at STAR Labs, especially with the loss of Harrison Wells as he was in Season 1. She’s going to find her new position and have different things to focus on. So I think that helps her get over her devastation from losing Ronnie again.”

Kreisberg added, “I also think there was a six-month gap… Even though it was a chyron, it definitely helped cover the time. And one of the things a lot of the actors have commented on to us [that] was something that we weren’t able to articulate necessarily in the writers room was, it feels like the show got more mature in the past year. Every one of the characters has grown up a lot and so everyone isn’t dealing with things in a very binary, raw, ‘I’m happy.’ ‘I’m sad.’ People are starting to grow up just like any of us do and start realize there’s complexities and there’s grief and happiness and there’s sadness and joy and you can still find your way.”

That means we shouldn’t expect to see Caitlin’s Killer Frost alter-ego any time soon, since Kreisberg sees it as more of a slow burn story. “We’ve been so blessed with the success that we’ve had that we’ve been able to… know that we’re going to be on for years and we were able to play the slow-con on some of these things. Just because one thing is happening fast, it doesn’t mean that something else is never going to happen, it just means that it’s being platformed,” he noted. “These things are being set up [and] it’s only in hindsight that you look back and you go, ‘that thing in episode whatever from early season 2, that tied to the thing that led to the other thing.’ There are plans for a lot of this stuff.”

Good Vibrations: Cisco’s own metahuman twist is developing a little faster, Kreisberg promised: “One of the things for him is this evil man said, ‘I’ve given you this gift’ and he’s seen what happens to the other metahumans. Good or bad intentions, they all go nuts and they all get locked up. Cisco’s really scared. He doesn’t see what the benefit is yet. He doesn’t see that it really is a gift and he doesn’t see that it’s a blessing and a power that can be used to help people. Right now, all he sees is the nightmare. That’s what’s really scary for him. It’s not like he woke up and he can fly. So not only is he scared about what it means to be a metahuman and all of that, but he also feels like he drew the short straw. ‘Barry got super speed and Ronnie gets to fly and me, I get these blinding headache nightmare visions of people getting killed.’ It’s not, at first blush, the most heroic way to step into the world.”

Cross-Eyed: Unlike last year’s “Arrow” and “Flash” crossovers, which were ostensibly about Oliver (Stephen Amell) helping teach Barry how to be a hero, this year’s mash-up is much more plot-based, not least because it serves to set the stage for “DC’s Legends of Tomorrow.”

“What Barry’s facing when he goes into the crossovers is part and parcel with what everybody’s going through in [episodes] five, six and seven,” Kreisberg said. “I will say that conceptually speaking, one of the ways we thought about these episodes in a macro sense was ‘The Flash’ episode this year plays more like an episode of ‘Arrow,’ and the ‘Arrow’ episode plays more like an episode of ‘Flash.’ We thought that was kind of the fun of these episodes. And what’s always fun about them is, this year, both Arrow and Flash are different, and I just mean the characters themselves – Thea’s on the team now so there’s all sorts of color combinations that are occurring.”

“And it’s exponentially bigger this year too because we factored in all the characters from ‘Legends,’ too,” Panabaker noted.

Monster Mash: Kreisberg hoped fans would be surprised – and thrilled – by the King Shark cameo, which he described as “a very expensive 30 seconds in the show. But our visual effects team are the best and they really love challenges like this. Armen Kevorkian, who’s the head of our team, got really excited, and it was probably the thing he sent me the most, like, ‘Check it out! Here’s how it’s coming!’ And I literally can’t believe that. That’s beyond feature quality and they realized it so well. Obviously we can’t afford to do an entire killer King Shark episode, but the fact that he’s one of Zoom’s minions … He’s just the latest in another line of [bad guys], which does mean there is King Shark on Earth-1.”

We’ll also get to see the return of Gorilla Grodd in season two, in which “Caitlin plays Fay Wray to Grodd’s Kong,” said Kreisberg.

“In season one you got to see just a tiny bit of her relationship with Grodd and you’ll get to see a little bit more of that and understand their connection from before,” Panabaker teased. “He has some unfinished business with her, I’d say.”

“The Flash” airs Tuesdays at 8 p.m. on The CW.