“I’m going to start crying,” says Wanuri Kahiu, waggling her hands and smiling. She’s telling me about a Twitter thread in which a bunch of people responded to the question “What was the happiest day of your life?” A young Kenyan woman replied: “Watching Rafiki with my mother and coming out.”

Rafiki is Kahiu’s new film – a gorgeous romance about two teenage girls falling in love in Nairobi. It’s a sweet movie, with a sex scene so tame you could watch it with an elderly relative. But in Kenya, a conservative society where 534 people were arrested for same-sex relationships between 2013 and 2017, Rafiki has been banned.

Over the past 12 months, Kahiu has been trolled on social media, threatened with arrest and suffered endless offensive comments, sometimes from members of her own family. “I have seen the vilest of comments come out of people I love,” she says. “It’s been incredibly challenging.”

At the same time, her career is flying. Days before Rafiki was banned, it was selected for Cannes. Now she has two Hollywood projects on the go: a sci-fi series for Amazon Prime, and a gig directing Millie Bobby Brown of Stranger Things, in a young adult drama produced by Reese Witherspoon, making her the first African woman to get a studio deal. One article bills her as “the next Kathryn Bigelow”.

We meet early in the morning in a London hotel. Kahiu flew in yesterday from Nairobi, where she lives with her husband, a cardiologist, and their two children. She looks a decade younger than her age (39), drinks peppermint tea, and speaks with focus and concentration.

Her troubles began in April last year, when the Kenya Film Classification Board demanded she edit Rafiki (which means “friend” in Swahili). “They felt it was too hopeful. They said if I changed the ending to show her [the main character Kena] looking remorseful, they would give me an 18 rating.”

Kahiu refused; the ban followed, the board claiming that the film sought to “promote lesbianism in Kenya contrary to the law and dominant values of the Kenyans”.

Ever since that ruling, Kahiu has felt threatened. The head of the classification board accused her of falsifying the script to get a licence to shoot the film. “He threatened to have me arrested but couldn’t because we never broke the law.”

What was the worst-case scenario? “Being arrested. Jails in Kenya are not the lap of luxury.”

She arranged a safe house in case authorities came after her or the actors. The language used by the head of classification was incendiary. “The attempt to normalize homosexuality is akin to air-conditioning hell.”

She successfully filed a lawsuit to get Rafiki shown in cinemas for seven days to make it eligible for the Oscars (in the end the Kenya Oscars selection committee didn’t choose it for submission for best foreign film). “All I did was make a fictional film. I’m literally just doing my job.”

The cases rumble on. She is suing the government for infringement of freedom of expression, and is back in court in June.

Kahiu was born in Nairobi to a paediatrician and a businessman. Her parents sent her to the UK for sixth form, boarding at the fee-paying Malvern College in Worcestershire. “That’s when I discovered I was black,” she says, laughing. “There were very few people of colour there at the time. But I made a really strong group of friends.” She read business management at Warwick before winning a scholarship to study film at California University.

She made a drama fictionalising the 1998 bombing of the US embassy in Nairobi, then a documentary about the Nobel Peace Prize winner and environmental campaigner, Wangari Maathai. But the film that shows off her ambition best is the short Pumzi, a 20-minute slice of Afrofuturism, set 35 years after the third world war has extinguished life on Earth.

In Africa, she has never experienced sexism as a female director, she says, in part because of the scarcity of directors of either gender, in part because woman have always been perceived as storytellers (“you tell children stories to keep them away from the fire while you’re cooking”). But Hollywood, too, “has been really receptive, surprisingly so”.

What she does find depressing is an expectation that, as an African film-maker, her work should deal with war, poverty and Aids. “It’s people thinking of Africa as a horrible, depressing, starving country. And therefore your work should be in reflection of that.” She rejects the idea that all art on the continent needs to be issues-led. What is needed are new visions of Africa, she says: “I think there are more instances of joy than remorse in Africa. If we don’t see more images of ourselves as hopeful, joyful people, we won’t work towards it. I truly believe seeing is believing.” To that end she co-founded Afrobubblegum, a collective supporting “fun, frivolous, fierce” African art.

For a 2017 TED talk, Kahiu came up with her own version of the Bechdel test, used to measure a film’s feminist credentials. In the Afrobubblegum test, a film must feature two or more African characters who are healthy, financially stable, having fun and enjoying life.

Colonialism still hangs heavy, she says. “When you’re swooping in and trying to save us, you’re not really adding any value. We’re not in that space any more. We [Africans] are coming in as contributors: 77% of the population is under 35. The global north should be really trying to find new levels of equal engagement with the global south.”

The tea has been drunk, photos taken and Kahiu will return to her room, not to sleep but to work on her Amazon script. On holidays, her husband tries to prise away the laptop. But it’s not that simple, she says. “When I think of a perfect day, work is involved. The only way I can process patriarchy, toxic masculinity, the only way that I can make sense of this film being in court, having people threaten my existence and my work, is by writing and creating. It’s the only way I feel I have control.”

Rafiki is released in the UK on 12 April