Mildred Hubble is a young witch attending Cackles Academy. She's called the "Worst Witch" because she's always caught getting into trouble.

In 1950s England, sailor Harold Guppy arrives in a small seaside town looking for his estranged brother. After a brief reunion, Harold finds a room for rent in the house of Mrs. Beasley, her meek husband and their teenage daughter, Joyce. It's clear that Harold has a troubled past, but his future is about to get more perilous yet. Despite the rosy surface, Harold soon finds himself in a tawdry sexual tug-of-war that won't end well.