Prenna James knows the rules. She must stick to her community. She must never reveal that she is from the future. And, above all, she must never fall in love. But in Ann Brashares’ The Here and Now, Prenna’s first-hand knowledge that the future of humanity is in jeopardy draws her closer and closer to Ethan Jarves, her classmate who’s silently protected her ever since he witnessed her materialize out of thin air.

The decision to write this time-travel narrative was a big one for Brashares, who admits that “it ended up being a lot more difficult than I originally anticipated.” The new novel is definitely a change in tone from the writer’s best-selling The Sisterhood of the Traveling Pants. It’s not an easy task to imagine not only how a person from the future could affect the present, but also how events in the present could change the future that Prenna was able to escape, along with approximately 1,000 other people who sought refuge in the New York City metro area. To figure out these multiple linear threads, Brashares says she spent “hours sitting with my head in my hands and my eyes closed just trying to follow each thread. I’d look up and realize I hadn’t written anything but at the same time I had done work that I absolutely had to do.”

The relationship between Ethan and Prenna wouldn’t be able to grow and thrive if the narrative was fully concerned with the science behind time travel and the “how” behind Prenna and her community’s decision to return to 2010 New York. “The how I couldn’t spend too much time on….The why was dramatically and narratively more interesting and important to me,” Brashares says. And the “why” is something much more familiar than the concept of time travel: Our near future is devastated by climate change, and with the rising temperatures and sea levels comes a terrifying plague carried by an everyday pest—the mosquito.

This concept of “why” is what brings Prenna and Ethan together, these “two young, energetic, reasonably idealistic people who have a sense that they actually have to do something.”

The novel began with the relatively uncomplicated idea that someone from the future falls in love with someone from the present—but this couple is not your usual teenage love story. “They’re facing these stakes that are bigger than them,” Brashares says. And while Prenna and Ethan show maturity far beyond their age and are capable of conceptualizing and solving problems which affect the future of the human race, they face an even more paralyzing issue in their future together: Their relationship is forbidden.

And while forbidden romance might not be a unique concept, the issues this couple faces aren’t of the “vampire vs. werewolf,” “cool kid vs. geek” variety. In this case “time is the enemy of romance,” says Brashares. This couple has such bad timing that it’s measured in decades rather than months or years. There is a lifetime between them, and yet they are together, driving through suburban New Jersey and trying to save the world.

Prenna and Ethan’s love story is powerful, but it is also marred by a tragedy bigger than their bad timing. The “why” of Prenna’s existence in 2010 is the largest reason for their relationship being forbidden. There is always the notion that “she could potentially could do him harm,” Brashares says. Prenna’s reality was one of plague, of people dying left and right without any way to stop it. She escaped that fate, but that doesn’t mean that it won’t happen again. And it will, if those with the knowledge of this terrible future don’t do anything to stop it.

Think their relationship sounds overwhelmingly difficult? Well, it is. But it’s an especially important relationship, in that these two characters struggle with the fact that the issues they face are much bigger than them. They are united out of love, yes, but also for a greater purpose. Brashares admits that she finds it “so fun” to write about relationships, especially young relationships at the time “when you’re making a lot of decisions that influence your life.”

But the decisions her characters make in this book involve the past, present and future, and humanity as a whole. That’s a lot for teenagers to grapple with, as well as for the writer and her readers. Ultimately, you have to “make your own rules for time travel and make sure they’re consistent and work for your narrative,” Brashares says. It’s easy to get lost in a swirl of temporal threads, and even Brashares admits “I could have gotten more than I bargained for, but I don’t regret that.”

“Sometimes time feels like this construct of our own consciousness and it’s almost asking to be mucked with,” she says. And Brashares did just that. But she did it not to instill in us a new theory about time or a warning that our atmosphere is heating up. After her mucking around, she leaves with us the simple idea that “love is bigger than time.”

Chelsea Langford is the assistant editor at Kirkus Reviews.