"A Boy Called Christmas" is a resplendent Santa Claus origin story with a star-filled cast, sumptuous visuals, and some melancholy details to keep it from being too sugary. Dame Maggie Smith plays the vinegary Aunt Ruth, a last-minute babysitter for three children who are still in mourning for their mother. It is Christmas Eve, but their father Matt (Joel Fry), sad and distracted, has decided they will skip the holiday this year. There will be no decorations or presents. After he leaves for a work emergency, Aunt Ruth begins to tell the children a story about a boy named Nikolas (Henry Lawfull), who lived a long time ago in a remote mountain cabin in Finland with his father, a woodcutter named Joel (Michiel Huisman). Nikolas, too, is in mourning for his late mother, who was killed by a bear, and each night he asks his father to tell him the story she used to tell, about a little girl who got lost in the woods one winter and happened upon a community of elves who loved and protected her until the spring thaw made it possible to go home, "her pockets full of chocolates." Nikolas has just one toy, a doll his mother made him from a turnip. And he has one friend, a mouse he names Miika and tries to teach how to talk. The king (Jim Broadbent) calls in his most loyal subjects, including Joel, to give them a challenge. He wants to bring hope back to his people, and he wants each of them to go off on a quest to find it. Joel joins a group to try to find the elves, just in case it's not a fairy tale. Like Matt, he leaves an aunt in charge. But Aunt Charlotte (look past the bad teeth and you will recognize Kristen Wiig) is not one to tell a cozy bedtime story. She's not even one to provide food or let Nikolas sleep inside the cabin. She is selfish and mean. When Nikolas discovers a clue to the location of Elfhelm, he and Miika (wittily voiced by Stephen Merchant) go off in search of his father to help him find it. Their search turns into a magical adventure through snowy mountains, with challenges both natural and fantastic. Nikolas helps a reindeer and names him after Lake Blitzen.