SPOILER ALERT: Do not read if you have not yet watched the series premiere of “The Passage,” which aired Jan. 14 on Fox.

Author Justin Cronin’s “The Passage” trilogy consists of more than two thousand pages chronicling a post-apocalyptic world in which an attempt at finding a new immune system booster turned people into “virals,” superhuman creatures that communicate telepathically and have a vampire-like thirst for blood. The majority of the action takes place once the virals are already dominating the world, but when adapting Cronin’s tome for her new Fox drama of the same name, executive producer and showrunner Liz Heldens knew she wanted to spend more time playing in the pond that was Project Noah.

“The thesis statement for this season is we’re examining the bad decisions and good intentions that lead to the end of the world as we know it. So we really wanted to start with the jungles of Bolivia, and Jonas Lear and Tim Fanning going in to find this mysterious virus that they think could hold the cure to all disease and then go from there,” Heldens tells Variety. “I felt like the audience needed to understand where this started. And it started because there was a man who loved his wife so much that he could not stand to let her go, and she was sick.”

The premiere episode showed how Fanning (Jamie McShane) was infected in the jungle and the lengths his partner (Henry Ian Cusick) went to to continue trials of the virus to hopefully use it to better the human race. They tested versions of the virus on convicted murders on death row, who seemingly had nothing to lose, but those inmates were all adults, and soon enough they realized they needed someone younger for it to synthesize the way they hoped. Enter Amy Bellafonte (Saniyya Sidney), a young girl who was put in foster care after her mother overdosed and who Project Noah “completely deemed disposable,” Heldens points out, by plucking her from regular life to use her in the next stage of their trials.

Heldens, who admits she had no idea the length of the novels or depth of the story when she first began reading Cronin’s novels, says the way she connected to the story was through the characters of Brad Wolgast (Mark-Paul Gosselaar), who was tasked with bringing Amy to Project Noah, Amy herself, and their relationship. Naturally, she wanted to “get to them as quickly as possible” in her version of the story, too.

“What I love so much about Justin’s book is that it’s really about human connection and love — just at every turn when people connect with each other and say, ‘I will be responsible for you,’ they’re better people, they have a better chance at survival,” she says.

One notable change she made was to age Amy up slightly, though, which altered the way the characters dealt with each other.

“I really wanted her to be old enough so that Wolgast had to work a little harder to get to know her and to gain her trust,” Heldens says. “And she had to be her own person with a point of view and agency, and be able to push back at him so there was more of a push and pull. Just to take a little bit of a page from ‘Paper Moon,’ it was important for her to have a voice.”

Heldens calls Amy’s tale a “coming-of-age story in the middle of this giant genre piece” and says, although she is not the apple-cheeked youth of the novels, the show will not play with the idea that Amy is not young enough for the experiment to work. “She’s still the one who is supposed to save humanity,” Heldens says. “We are following the spirit of the book, and I think we are following pretty closely the big movements of the book.”

In the novels, Amy ages about five years for every 100 that pass, which Heldens says provides a “cushion” for the show and how Sidney is growing up quickly in real life.

This means that Amy will age in the show similarly to how she does in the novels, and, of course, at some point, the virals will break out of their boxes and end the world as the characters know it — but that won’t be for a while. First, Heldens wanted to get her audience “really invested” in everyone, so she built out a flashback structure to dive deeper into the backstories of characters ranging from virals such as Shauna Babcock (reimagined as a woman for the television show and played by Brianne Howey) and Anthony Carter (McKinley Belcher III), to the men that started it all, Fanning and Jonas, and eventually to Wolgast, too. (Heldens admits if the show is a multi-season success, they won’t be able to flashback to Amy later on, as they are not banking additional content with her looking the way she does now, but she isn’t concerned about that because “there are many other stories to tell.”)

Front-loading Heldens’ version of “The Passage” with details about these individual characters meant “pillaging” from all three books in some cases.

“It’s such a nice luxury on a TV show version … to get to know the virals and to understand that they were people with lives and stories, and they were all dealt bad hands,” Helden says. “We’re exploring Jonas Lear’s relationship with his wife, and we’re exploring the fact that Tim Fanning was in love with [Jonas’] wife … and then later you learn the details on the death of Wolgast’s daughter and how he came to get his job at Project Noah, which speaks to his relationship with Richards.”

Heldens says she wanted to start diving into characters’ backstories with the virals because they “have no voice.”

“As far as we know at Project Noah, they cannot speak, and we think they have lower brain function, so it seemed like the most surprising and revelatory place to go first — ‘Let’s see what’s happening behind the curtain with these people,'” she says.

On the flip side, Wolgast is one of the characters whose pasts get explored last because of how “front and center” he is in the present day story. “It felt like his story was compelling enough that we could wait a little bit and have him go through hell at Project Noah before telling the story of what happened with his daughter and why it’s so important for him to stick by this little girl,” Heldens says.

As for when the show will flash forward to the post-apocalyptic trials and tribulations that make up the majority of Cronin’s tale, Heldens says her approach was that because it was a big reveal in the book — “You’re reading and you’re like, ‘Can one girl save humanity? OK she’s the key to something, so she’s going to fix it’ and then you turn the page and it’s 100 years later, you’re like, ‘Oh s—!'” It has to be a “big, crazy moment” in the show, too.

“How do we build a world in the future that is compelling and believable? It’s really another pilot, in a way,” she says.

But the richness of that world is a “luxury,” rather than something overwhelming to Heldens. And even though the look of the show may change drastically when it reaches that story point, the themes are still the same.

“You can find redemption within connection,” she says. “We certainly have held onto that in the writers’ room and for a writer who loves character, that is just a lovely gift [Justin] gave us.”

“The Passage” airs Mondays at 9 p.m. on Fox.