The Secret Series is a collection of exciting action-adventure classics that relate trilling adventures of Arnold children, older Peggy and younger twin siblings Mike and Nora, who live with a harsh aunt and uncle after their parents are thought to have been killed in a plane crash.
"The Secret Island" – Aided by Jack, an orphan they have befriended, the Arnold children run away to an island on a nearby lake, and together they make a new home constructed with the branches of a willow tree.
"The Secret of Spiggy Hole" – The Arnold children discover that their parents are alive and reunite with them. The family goes on a holiday at the rugged Cornish coast with a family friend, Miss Dimity. Children get drawn into a kidnapping case as they discover a boy named Paul, Prince of Baronia, being held prisoner in a castle tower nearby.
"The Secret Mountain" – Captain and Mrs. Arnold are flying off in their plane, the White Swallow, and apparently have crash-landed in a remote part of Africa. The Arnold children, home from school and in the care of a family friend, Miss Dimity, team up with their friend, Prince Paul of Baronia, and go flying off in his royal plane to Africa to rescue their parents.
"The Secret of Killimooin" – Prince Paul invites the Arnold's to his palace in Baronia for the holidays; but it is so hot there in the roasting summer that the entire royal household adjourns to their castle up in the mountains. Bands of robbers are marauding the countryside, terrorizing the local people, and Paul is incensed, and determined to find the robbers and have them detained.
"The Secret of Moon Castle" – The Arnolds are invited to spend some time in the castle with Prince Paul's family. They are all soon puzzled by strange events happening in the castle: odd noises that occur, eyes in portraits that shine mysteriously, lights that shine in tower rooms, despite being locked tight.