At an unknown point in the near future, a woman is giving birth. As her contractions start, her home in London is flooded and, as her baby is born, it becomes clear that a climate catastrophe of biblical proportions has begun. A sea of water invades the city as the woman takes her first postpartum, post-apocalyptic pee, then flees for higher ground with her newborn in a car seat.

So begins the story of a new survival film starring the British actor Jodie Comer. The End We Start From goes on general release in UK and Irish cinemas from Friday and has a star-studded cast: Benedict Cumberbatch, Joel Fry, Gina McKee, Nina Sosanya and Mark Strong all appear alongside Comer.

For the author Megan Hunter, who wrote the extraordinary 2017 novel – her debut – which the film is based on, the plot was obvious: “I’ve always had an apocalyptic imagination – I remember, even as a child, dreaming about the sea overwhelming the Earth,” she says.

After having two children in her 20s, the 39-year-old found herself writing lots of poetry and short stories about motherhood in her spare time. “Then I had this idea to write a story about a woman giving birth in the future. And it seemed inevitable to me that she would be facing a climate disaster.”