Danish star Trine Dyrholm takes on the “bad mom” trope in “Birthday Girl,” which has its world premiere at the Zurich Film Festival, where a girl’s 18th birthday party goes horribly wrong.

Directed by Michael Noer, it sees her character, Nanna, trying to impress estranged daughter (“As in Heaven” breakout Flora Ofelia Hofmann Lindahl) with a cruise to the Caribbean. But when the girl is assaulted, and no one believes her, Nanna needs to step it all the way up.

“Birthday Girl” was produced by Matilda Appelin and Rene Ezra for Nordisk Film Production, with TrustNordisk handling sales.

“I saw her as a mother who wants to be her daughter’s girlfriend. They are on the biggest party boat she could find, because they haven’t seen each other that much and she wants to make up for all that. She is fighting for this closeness and then they share this terrible experience,” says Dyrholm, on her way to Poland to work on Magnus von Horn’s upcoming “The Little Seamstress.”

As long-haired, long-nailed Nanna starts to investigate, to the horror of the crew and party-loving guests, nobody takes her seriously either.

“I found it very important that she looks like this. We always place people in boxes, but that’s how she expresses herself, with these push-up bras and false eyelashes. It can be a mask, but it can also be freeing. That lady is a survivor,” says Dyrholm.

“On these cruise ships, you can’t get out and there are no rules, because these are international waters. When something like this happens, you are on your own. The scariest thing is when they get an upgrade. They put them in a bigger cabin, hoping everyone will shut up. It’s a good example of how our society treats this topic.”

Still, victims of sexual assaults have been consistently speaking out, no longer interested in silence.

“We are finally talking about it in a real way. You can’t just push it aside. My character also experienced something she couldn’t really talk about when she was younger. Now, she wants to make it right for her daughter, because she wasn’t able to do it for herself,” she adds.

“I think that for Trine, it was interesting to play a part when she doesn’t have to be a saint,” says Noer.

“It was a gift to have these two actresses and let them be experts on what it means to be a woman in certain situations. I don’t know how it feels when men talk about how drunk you were or the length of your skirt. Nobody is going to ask me how drunk I was, unless I am involved in a DUI.”

Making the film made him aware of his own prejudices, he admits.

“Women can be dismissed because of how they look. ‘Put more clothes on, lose some weight.’ We talked a lot about gender, but we also talked about age. There is this idea that if a woman is ‘misbehaving,’ she better be as young and good-looking as Taylor Swift,” he observes.

“In the film, I called this company ‘Coco Cruise,’ after my daughter. I tried to make a movie that she can hopefully watch one day and say: ‘Ok, my dad tried to understand something.’ Maybe it’s because I am a bit of an idiot myself, but I gravitate towards characters who are lost and make mistakes. I myself had misconceptions towards Trine’s character, but she is doing the best she can. Unlike other people on that ship.”

Noer, also behind Rami Malek starrer “Papillon” and drama series “Prisoner,” is no stranger to claustrophobic environments.

“It’s not that I have some kind of a fetish, but I am interested in humans colliding with their surroundings. I like genre movies, I like this ‘ticking clock’ thing. It was interesting to take someone who is a ‘bad mom,’ fighting to be a good mom, and put her under such pressure,” he notes, mentioning a New York Times piece that started it all.

“I come from a documentary background, so almost all my ideas begin with an article. This one compared number of sexual assault cases in Miami hotels and these ships. They are not rare, it’s just rare for them to resurface. There are no real police [on board]. Everyone’s paid by that company,” he says.

“Even in ‘Succession’ there is a whole subplot about lawless cruise ships and yet we don’t do anything about it. The tagline of the movie could be: ‘Why can’t you just leave it on the boat?’ But some things you can’t ‘leave on the boat.’ We need to talk about them.”