Based on the 1973 children’s book by author John Bellairs and illustrator Edward Gorey, and set in the mid-20th century, “The House with a Clock in Its Walls” follows a young orphan named Lewis, who moves in with his uncle, Jonathan (Jack Black), after the death of his parents. Shortly after arriving, Lewis discovers that his uncle is a warlock, his neighbor, Mrs. Zimmerman (Cate Blanchett), is a witch, and that the house they live in contains a magical clock hidden in its walls (shocking, I know).
However, it’s no ordinary place: The building is a character in and of itself, with furniture, stained glass windows and other aspects that come to life as well as a mysterious tick-tocking heart at its center that has the power to end the world. When Lewis inadvertently wakens the dead, he begins to learn a lot more about the wondrous (and occasionally dangerous) world around him — including wand-waving neighbor Florence Zimmerman (Cate Blanchett).
The house’s previous owners were sinister warlock, Isaac Izard, an orphaned former friend of Jonathan’s who was traumatized while fighting in World War II, and his equally wicked wife Selena. They had apparently hidden a clock within the walls of the house before they died, and Jonathan has been trying to find it and discover its purpose. Despite Jonathan’s cautions, Lewis begins teaching himself magic.
On his first day at his new school, Lewis meets Tarby Corrigan, who is nice to him during physical education class and then later asks Lewis to help him with batting practice and they become friends. Tarby asks for Lewis’s help to campaign for class president. However, when Tarby wins the election, he abandons Lewis. Later, a classmate, Rose Rita Pottinger, tries to explain to Lewis that Tarby didn’t actually like him and was just exploiting him to win the election, as he does the same thing every year, but Lewis dismisses this. Additionally, Lewis’s mother visits him in his dreams. When he laments that Tarby does not see him as a friend, she suggests that he use a spell from a forbidden book to impress him. On Halloween night, Lewis performs a necromancy spell with Tarby in a cemetery, which accidentally summons the corpse of Isaac from his grave.
Lewis spots Isaac in the window of the house of neighbor Mrs. Hanchett. He “rescues” her from Isaac and brings her across the street to Jonathan’s house, but before he can find Jonathan, Isaac confronts them at the front door. He reveals that Mrs. Hanchett is really Selena, who previously killed Mrs. Hanchett, took her place, and used her bones to make the clock’s key. It was also Selena who took the form of Lewis’s mother to persuade him to summon Isaac.
Isaac explains that his hidden clock’s blueprints came from the demon Azazel, who had given them to him as he sought relief from the visions of horror he had witnessed during the war. The true purpose of the clock is to “reverse” all of human history back to the very beginning, thereby rendering humanity extinct and enabling a new world to be created in Isaac’s own twisted image. Jonathan, Florence and Lewis are then magically expelled from the house as the evil couple initiate their ritual.
Using the Magic 8 Ball to learn the location of the clock underneath the boiler room, the three return. Florence stays behind to defeat a snake guarding the room while the others pursue Isaac. The clock’s power transforms Jonathan into a baby when he attempts to disarm it, except for his face which he’d shielded with enchanted cards.
Lewis consults the Magic 8 Ball, which says “Say goodbye”. He realizes that he has to let go of the pain of losing his parents to harness his true power. He breaks the clock by dropping the ball, which blocks the clock’s gears. Lewis harnesses the clock’s magic, undoing the ritual and unleashing it instead on Isaac and Selena. They fall, de-age, and are disintegrated, leaving only their clothes.
Lewis returns to school with more confidence and gets back at Tarby and his friends by magically bouncing a basketball off their faces and into the basket, impressing his other classmates. He then openly befriends Rose Rita, who appears to have a crush on Lewis. At the end of the day, Jonathan and Florence are shown picking up Lewis, the three of them now living as a regular family.