This rather lovely film tells the story of a man called Billy Mail, and his otter – a pup he rescued after spotting it half-starved on the pontoon bobbing in the Atlantic at the bottom of his garden in Shetland. Billy called the otter Molly, and started feeding it. On the voiceover his wife, Susan, says Molly took over their lives, and she’s not kidding. By the end of the film, Molly is living in a handbuilt miniature bothy in the garden, dining on haddock; we get to feast on Shetland’s epic scenery, beautifully shot by wildlife cameraman and National Geographic photographer Charlie Hamilton James.

Otters are apparently extremely shy, so Molly must be desperate, Billy reasoned when he found her in March 2021. He bought a book about otters, which was no help at all trying to raise a pup, so he made it up as he went along. “I felt a bit daft sometimes,” he says over footage of himself filling a tub with brightly coloured plastic balls for Molly to play in. But, like a good mummy otter, his plan was to get Molly to the point where she could go it alone and survive the winter. Susan wonders if the relationship flipped from Billy keeping Molly going to the other way round.

The couple both talk briefly about Billy’s grief at not having had children, leaving him with a lasting sense of loss; director Hamilton James is polite in respecting their privacy, and doesn’t prod. There’s a twist coming when a bruiser of a male otter shows up. But really, this is a film of gentle pleasures, watching the couple’s house gradually becoming otter crazy. A novelty mug on the kitchen drainer reads: “You’re my otter half.” I pitied the couple’s longsuffering dog, frequently filmed nosing a ball forlornly, clearly feeling dejected and ignored.

• Billy & Molly: An Otter Love Story is on Disney+ now and National Geographic from 15 November