In 1930s England, 17-year-old Cassandra Mortmain and her eccentric family struggle to survive in a decaying English castle.

As part of a drunken bet with her sister Felicity, happily married Alice sends an anonymous Valentine's card to her husband Sam to see if he hides it. When he does, what was a prank leads to a series of events and revelations that may put her marriage at risk, and leaves her looking for answers.