There are two different films vying for attention inside A Monster Calls. One is a tense, fevered, live-action, gothic fantasy about a 12-year-old boy coming to terms with his mother's mortality. The other, a sequence of childlike animated fairy tales, interrupts the first film in fits and starts.

Put them together, and you have a deeply affecting, if uneven, coming-of-age yarn from the talented and versatile Spanish-born director J.A. Bayona (The Orphanage), who has also been tapped to direct next year's Jurassic World sequel.

Adapted by children's author Patrick Ness from his and Siobhan Dowd's 2011 best seller,  A Monster Calls is set in a verdant, if perpetually downcast, corner of suburban Britain. It centers on an isolated, awkward boy named Conor (Lewis MacDougall) who has to fend for himself and take care of his single mom as her battle with cancer grows steadily more hopeless.

MacDougall dominates the film with a modulated performance as Conor, a budding artist who is mercilessly bullied at school.

The boy's only companions are his drawings and an ancient yew tree that stands alone behind his house. Sure enough, the yew tree comes to life, transmuting into a dark, destructive giant.

But while he's a terrifying monster, this creature has the soothing, avuncular voice and manners of Liam Neeson. He comes to Conor every night to tell him fairy tales.

The usually vivacious British actor Felicity Jones, who has shot to international stardom with the success of Rogue One, gives a muted, haunting performance as Conor's mom, Lizzie. She registers on-screen as a heartbreaking absence.

Jones' unearthly, ethereal turn is nicely counterbalanced by Sigourney Weaver's imposing performance as Lizzie's mother, an erstwhile practical and no-nonsense woman reduced and silenced by grief.

At its best, A Monster Calls is a beautiful, dark fable about Conor's inner life that somehow manages to capture the boy's complex, ambivalent feelings about the possibility of losing his mom. He's virtually suffocated by a mixture of fear, guilt, shame, and grief.

Director Bayona began his career in horror, and he uses the raven-black tropes of the gothic brilliantly. He's far less successful when it comes to the film's animated sequences, which illustrate the monster's fairy tales. Their visual style is far too young, too unsophisticated, too bright to carry the story's weighty themes.

Despite this incongruity, A Monster Calls is an engrossing tragic fantasy, sustained by genuine sentiment.