A Monster Calls is a British adaptation of the book of the same name by Patrick Ness, and explores the themes of bereavement and isolation. It follows Conor, a young lad whose beloved and devoted mother is seriously ill. As her condition worsens others take over his care, but he has little in common with his meddling grandmother, and his father has resettled thousands of miles away. Conor finds a most unlikely ally when a monster appears at his bedroom window one night.

In keeping with the book, the film has several fantastical elements.

There are scenes of moderate threat as a huge tree comes to life and appears to directly threaten the main character, a child. There are also repeated images, in dream sequences, of a collapsing building and crumbling ground which leaves the boy's mother clinging to a ledge above a chasm.

Though the scenes are intense, in the context of the film which is looking at difficult ideas from a child character's point of view, the BBFC Compliance Officers who viewed the film suggested these ideas could be contained at 12A, where moderately intense scenes are acceptable.

They also noted that the film was likely to have a real emotional impact on the audience. There are several scenes of emotional distress, in which Conor has to come to terms with his mother's condition as the film explores the difficulties of her terminal illness and Conor's attitude towards his bereavement.

There are also scenes of bullying in a contemporary school setting. Though the violence in these scenes is relatively mild, a young audience is likely to identify and sympathise with Conor.

The BBFC rated A Monster Calls 12A, which means over 12 year olds can watch the film, but any children under 12 must be accompanied by an adult to see it in the cinema. The Compliance Officers cited 'moderate threat, scenes of emotional distress' as the key reasons for this decision in the film's BBFCinsight, which is published on the main BBFC website, and the free BBFC app, together with a longer description of the classification issues. BBFCinsight is particularly important for parents, and is designed to help them make the most appropriate viewing decisions for their own children.

Following the film's classification and release in cinemas, the BBFC's Advisory Panel on Children's Viewing – made up of experts in the fields of education, child development, and child protection – discussed the film and unanimously agreed with its 12A classification.

More information is available about the BBFC Classification Guidelines, the printed criteria used by BBFC staff when making age rating recommendations. The Guidelines are based on extensive public consultation and the law.

The BBFC's Education team selected A Monster Calls as one of the films they introduced at the 2017 Into Film Festival.