A Monster Calls is deeply cathartic, the kind of film that leaves you mesmerized and gutted at the same time. It’s emotionally explosive with a strikingly simple plot: A young boy uses fantasy to reckon with the unbearable tragedy of losing his dying mother.

Tenderly directed by J.A. Bayona and based on the award-winning novel by Patrick Ness (who also wrote the screenplay), A Monster Calls follows 13-year-old Conor O’Malley (newcomer Lewis MacDougall) as his world starts to fall apart. Increasingly conflicted with the people around him, he begins to disappear into a fantasy realm where a large, fire-breathing yew tree monster (voiced by Liam Neeson) helps him come to terms with his grief through a series of stories. The fourth story, the monster tells the boy, is Conor’s to share — it’s the ugly truth he’s been running from.

“Kids, they feel loneliness,” Bayona told MTV News. “They feel rage. They feel lots of emotions that they don’t know how to channel. This film addresses that, and it addresses that using fantasy, which makes it accessible. You look at Conor and it’s extraordinary, the complexity of the journey that he takes in this story.”

In the wrong hands, A Monster Calls could have been manipulative melodrama. But Bayona’s ability to balance fantastical imagery with unsettling catharsis turns the film into something more thrilling. You know what’s going to happen to Conor’s mother, yet you’re transfixed by his momentous journey. For Ness, who held on to the film rights for years and wrote a screenplay before anyone was attached to direct, it was important to find a filmmaker who understood the sincerity of Conor’s experience.

“I want stories that make me feel stuff, and they rarely do in a real way,” Ness told MTV News. “I didn’t want convenient, pretty crying [on-screen] because that felt untrue. ... It would cheapen it. So I thought, Really push to get the hard truth of it because if you do that, it becomes sad without being bleak. And sadness is a true thing.”

Ness’s challenges writing the screenplay derived from how unrelenting a film as a storytelling medium can be to execute — book readers can take a break where filmgoers can’t — and from how the book reads from Conor’s point of view, meaning characters like his father and grandmother are written as how he interprets them.

“Film is relentless,” Ness said, “and you want to be captured by it. But because of how emotional the material is, you have to be really careful. You can’t batter an audience or make them feel strong-armed into feeling something.”

Bayona wisely decided never to reveal to the audience whether The Monster is real or a figment of the boy’s colorful imagination. There are clues that would suggest both but nothing is definitive. Ness took a similar approach when writing the book. “It’s ambiguous, and I’m happy with it,” Ness said. “In the book, there is very specific evidence both ways. There are tree needles on the floor, so you think he’s real, but then the room is repaired, so he’s not real. I like that.”

Not every character from the book made it into the film, but because the book is so internal, Ness added certain elements: Conor’s mother, for example, is an aspiring artist who gave up her dream of attending art college to raise her son, a talent and impulse that lives on in Conor. These details led to a heartfelt new ending for the film that leaves room for interpretation.

“I told Patrick from the very first moment, we need to find some light at the end of the journey,” Bayona said. “The most important idea that we worked on together was the idea of legacy — how someone’s legacy can keep them alive, how art can defeat death. This is what I wanted to do at the end of the story, in a very romantic way. I wanted to defeat death and overcome the situation that Conor is going through.”

“You cannot leave the audience in the dark at her death,” Ness added. “We have to have a moment where we know that Conor has made it through.”

As proven with A Monster Calls and his award-winning Chaos Walking trilogy, Ness understands that teens don’t need to be sheltered or patronized, but treated like adults. His books expose the ugly, unbearable truth about the world: that it can be woefully unfair, especially for young people who are often given many of the responsibilities of being an adult but none of the privileges.

“For whatever reason, we conveniently forget how difficult it is to be a teenager,” Ness said. “The worst thing that you could say to anyone is, ‘You’ll grow out of it.’ Because even when that’s true — and it’s certainly not always true — you do not live in a theorized future. You live in right now. To diminish that daily experience as something to be dismissed I think is cruel. ... It’s the time in our lives when we become complex, and that’s what adulthood is: complexity. So why do we treat it so callously? That’s the whole point of A Monster Calls.”

Conor may not be dealing with the end of the world, but he’s dealing with the end of his world — and Ness and Bayona never let you forget that.

“All of my books have been, not even intentionally, [about] what do you do after the end of the world? How do you survive?” Ness said. “Because we end up surviving it all the time.”