It's the first day of the summer holidays and Harriet, Jake and Matthew are off to see Grandmother, who lives in Cliff View Cottage in the seaside town of Ormsthorpe. They're looking forward to spending their days on the beach, cliff-climbing and exploring the rock pools. But then one stormy night, when the moon is full, something quite strange happens. They are transported to an ancient village, where they meet Olaf and Sigrid and learn all about the ways of the Vikings. Their new friends explain that their own children have been kidnapped and enslaved by terrifying raiders. And they need their help to rescue them. But first, they must come up with a plan... Join Harriet, Jake and Matthew as they disguise themselves as Vikings, sail the stormy seas by longboat and embark on a dangerous undercover mission.