The Arnold children discover that their parents are alive and reunite with them. Also, their parents adopt their new friend Jack, who is an orphan. The family goes on a holiday at the rugged Cornish coast with a family friend, Miss Dimity. In a dangerous adventure, children get drawn into a kidnapping case as they discover a boy named Paul, being held prisoner in a castle tower nearby. Paul turns out to be the Prince of Baronia, a small, remote kingdom and kids become firm friends with him.