Spoiler warning: Do not read on unless you’ve seen “The Flash” Season 2, Episode 11, titled “The Reverse Flash Returns.”

Barry Allen (Grant Gustin) has already had to make a number of painful decisions during his career as The Flash, but the choice not to save his mother from the Reverse Flash is one that will likely haunt him for the rest of his life — even though he made that sacrifice for all the right reasons.

So when the Reverse Flash returned in Episode 211, capturing him offered Barry the possibility to change the timeline, since this version of Eobard Thawne had yet to kill Barry’s mother, Harrison Wells or many of his other future (past) casualties. But as executive producer Andrew Kreisberg pointed out in a conversation with Variety, “as so many time-travelers and people who’ve been confronted with time paradoxes before them have discovered, time does not want to change.”

Keeping Eobard locked up threatened to erase Cisco (Carlos Valdes) from their timeline, so Barry once again had to make a painful choice, letting go of his archenemy in order to save his friend. That also played out in Barry’s relationship with Patty (Shantel VanSanten), after she chose to leave Central City. She finally figured out Barry’s secret identity as The Flash, but his reluctance to trust her with the truth proved to be the final nail in the coffin of their relationship.

Below, Kreisberg breaks down the episode’s surprises and what’s coming up in the next few installments.

Why was it important for Barry to make such a difficult choice again after he was faced with a similar sacrifice last season?

Barry’s challenge this season was encapsulated in that video message that Wells/Thawne left at the beginning of the year where he said, “I’m gonna give you the thing you want most in the world but you’re still not gonna be happy, because you’re never going to be happy.” And Barry’s discovered that even though his dad is out of prison, not having him around isn’t making him happy, and things are on the skids with Patty, and now… he’s being given a chance to [defeat his greatest enemy] again and he didn’t take it. So it’s really all driving Barry towards feeling “if I can just defeat Zoom, then I’ll be happy.” Because Barry doesn’t realize that he’s a character in a TV show and he’s going to be facing multiple villains year in and year out. [Laughs.] So he thinks there’s an end goal that he can reach, and if he could just reach that end goal, he will have healed himself and replaced all the things that he’s lost.

Barry resigned himself to losing Patty this week — was that another way to further narrow his focus?

Wells has challenged Barry and said “you’re never gonna be happy,” so to get Barry to a place where he feels like stopping Zoom is the only thing he can do, we wanted him to start losing things in his life that made him happy. We loved working with Shantel and she’s been such a boon to the show, but we felt like having Barry not be able to maintain this relationship was a step towards him feeling like his only recourse towards being happy was defeating Zoom. But we very consciously did not kill her, so there’s always the possibility that she’ll be back.

Episode 12 will focus on Wally (Keiynan Lonsdale) and Iris’ (Candice Patton) relationship. What else can you preview?

It’s a really interesting episode because Wally’s drag racing starts to become a real issue for both him and Joe. And Iris takes it upon her journalistic self to insert herself into the story. It’s really Iris driving the action in that episode and there’s some great emotion between Iris and Wally and a great scene between Joe (Jesse L. Martin) and Wally. As we said at the beginning of the year, because so much of the Allen family drama from last year was solved, it’s really been about the West family and these episodes with Wally and Joe and Iris all getting to know each other and trickily trying to navigate this insta-family comes to a head in that episode. 10, 11 and 12 are sort of the Wally-Joe-Iris trilogy and it’s a really great episode directed by Rachel Talalay who directed the last two years’ two-part season finales on “Doctor Who,” so I was obviously very excited to work with her and she is [so] talented, amazing and delivered a knockout episode.

In Episode 13, we’ll take a major trip to Earth-2 — how much of a fixture will that alternate universe be in the back half of the season?

13 and 14 are a two-parter. 13 is called “Welcome to Earth-2” and 14 is “Escape from Earth-2,” so the majority of those episodes will be taking place on Earth-2 and there’ll definitely be a couple more visits throughout the course of the season. We liken it to the time-travel plan that we had for last season which was to slowly let people in, and then once they understood it, to really start to embrace it. It was around this time last year that we did the big time-travel episode so we feel like we’re treading in those waters again where we’ve been hinting at Earth-2 and you’ve seen fleeting glimpses of it, but now the audience understands what it means and they’re able to step into it.

King Shark will return in Episode 15, which will also feature a crossover visit from “Arrow’s” John Diggle (David Ramsey) and his wife Lyla (Audrey Marie Anderson). Diggle has struggled with adjusting to metahumans more than anyone, so how does he adjust to the insanity of Central City on a protracted visit?

He’s coming with his wife, which is a whole different thing. Lyla’s seen it all, Lyla works for ARGUS, you’d have to really pull something out to ruffle her feathers, so part of the fun of this episode is, Dig is still not over The Flash and not only has she seen all the things that we’ve never seen, we’ve all seen her experience The Flash and she’s used to it by now, so there’s a funny Mr. and Mrs. Smith kind of banter between them where she’s like “John, get over it. He’s really fast, I get it.” But as always, whenever we have a crossover, it’s more than just a gimmick, and Barry’s in a little bit of a tough place and Dig is a soldier and somebody who’s been through a lot and can be there to offer Barry some advice he wouldn’t necessarily have come up with on his own. It’s a fun episode. We were as blown away by King Shark as everybody else was, and really, the hard part is building them and creating them, characters like King Shark and Grodd, and once you’ve done that, if you figure out the right story and the right way to tell it, you can really do something spectacular. We didn’t want any of these episodes to feel like you could miss one of them, so how are all these mid-winter episodes special? We had an amazing new villain last week with The Turtle and this week we have the Reverse Flash and we’ve got the Earth-2 stories and then you have King Shark, and we’ve tried to make sure that every one of these episodes has something amazing in it and a reason to tune in.

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