Some movies find themselves wading up to their thighs in a treacle-lake of their own pointlessness. One such is Sybil, directed and co-written by French film-maker Justine Triet, making her Cannes competition debut. It is a muddled, silly comedy-drama starring Virginie Efira as a psychotherapist ironically named Sybil (no one could be less like the fabled ancient Greek soothsayer).



Ten years previously, Sybil was a bestselling author who went for a big career change for personal reasons: she had a painful breakup with her partner Gabriel (Niels Schneider), with whom she has a child. Now she is reasonably happy with a new partner, Etienne (Paul Hamy), though she still struggles with alcohol addiction. She has had another child but is yearning to return to her literary vocation. One of her patients has a fascinating, troubling personal life which may cure her writer’s block. Margot (Adèle Exarchopoulos) is a movie actor having an affair with her co-star, Igor (Gaspard Ulliel). She is pregnant with his child, but he is in a relationship with their director Mika (played by Sandra Hüller, from Maren Ade’s Toni Erdmann). They are incidentally shooting their film on the island of Stromboli, perhaps as a homage to Roberto Rossellini and Ingrid Bergman.



Against all the therapy rules, Sybil starts covertly recording their conversations to use as raw material for fiction, and then has a compulsion to intervene in Margot’s life, trying to shape it like an omniscient narrator. And, of course, Margot’s pregnancy has a painful personal resonance.



Almost any of the constituent parts of this set-up could work as the basis of a movie. The idea of a shrink secretly converting her patient’s life into fiction is certainly promising. But the film’s focus is hopelessly split among all the other things going on. Sybil’s backstory is frankly overblown, unconvincing and uninteresting – although Laure Calamy (from the Netflix French TV hit Call My Agent) has a funny cameo as Sybil’s sharp-tongued sister, who gives her kid a tutorial in emotional manipulation. All the tautness and tension that is needed for the main Margot/Igor/Mika/Sybil story is dissipated with other stuff - and even the main story is complicated by Mika, who is not a very well-written character – and sadly Hüller is not especially good in the part.

Fundamentally, Sybil is not funny because it is not convincing, and some of the acting is not of the highest order. Efira’s “drunk” turn is something she may wish to omit from her showreel.

• Sybil screened at the Cannes film festival.

