Henry, Jessie, Benny, and Violet Alden meet Mike Wood and a strange chain of events begins. Mike's house is mysteriously burned down, a man in a blue hat appears, and the Alden's uranium mine is in danger of being sabotaged. The four youngsters solve the mystery.

The Alden children and their grandfather are hiking down from Flat Top when a rockslide reveals a cave with a big secret.

A young Canadian visitor helps the Alden family solve a mystery involving a lost coin collection.

The Alden children are helping their new neighbors build a tree house when they discover an attic window that not even their neighbors knew about.

A friendly writer with a metal detector, the mysterious inhabitants of a tower house, and the discovery of a locket in the sand are ingredients for the Aldens' latest mystery.

Henry, Jessie, Violet, and Benny hav3e promised Grandfather a vacation from mysteries during their island visit. But the romise proves hard to keep when things start to go missing. Then an old fisherman tells Benny of the island's ghost horse, and late that very night benny sees a mysterious stallion galloping down the beach. Could the ghost horse have something to do with the disappearances?

One warm night four children stood in front of a bakery. No one knew them. No one knew where they had come from. Henry, Jessie, Violet and Benny are orphans.They discover an old red boxcar that provides them shelter from the storm.

The Alden children have another mystery to solve when they visit a castle and must figure out which of the guests has stolen the Stradivarius violin.

The Alden children are thrilled when a famous orchestra comes to their town to perform, and when an original Mozart score turns up missing, they solve the mystery of its disappearance.

When the Aldens take up the case to investigate if a catastrophic oil spill that affected hundreds of species could have been prevented, they are led on an adventure to help other endangered animals around the world.