Danish film-maker Niels Arden Oplev, director of the original Girl With the Dragon Tattoo with Noomi Rapace, has written movingly about the true story behind his very personal new film. It is inspired by his sister who has schizophrenia, perhaps triggered by heartbreak she experienced in her personal life as a teenager working in her gap year in France, but who in middle age went on a cathartic healing journey back to that country with her sister and brother-in-law.

Sofie Gråbøl (star of the Scandi noir TV hit The Killing) plays a fictional version: Inger has schizophrenia and lives in residential care; she is about to take a bus trip to France with her caring, if nervous, sister Ellen (Lene Maria Christensen) and Ellen’s bullish, good-natured husband Vagn (Anders W Berthelsen). It is a tense experience, because Inger still talks frankly about the invisible creature called “Goldensun” who speaks to her and encourages her to self-harm, and also because she also has a habit of making loud, sexually inappropriate comments, to the uptight and heartless disapproval of a mean guy on the bus who would prefer not to be anywhere near this person. But this man’s sweet 12-year-old son, inevitably more innocently compassionate, makes friends with Inger.

Unfortunately, the film is saddled with treacly liberal good taste, and watching Gråbøl’s elaborate impersonation of someone with schizophrenia, freaking out a bad person on a bus … well, it’s impossible not to think of Lars von Trier’s satire The Idiots from 1999, about an anarcho-situationist prankster cult of people pretending to have cerebral palsy in public places to confront and discomfort the caring bourgeoisie. For all that it is based on a true story, this film’s characterisation and narrative are massaged into a kind of sentimental drama. It rings false.

• Rose is in UK cinemas and digital platforms from 28 June.