The adventures of the four Alden children as they spend a summer vacation on their very own island, purchased by Grandfather's father years ago.

The Aldens climb a mountain and find a mystery! The hike up Old Flat Top was only supposed to take a day, but a dangerous rockslide changes everything to reveal a hidden cave that just may hold a legendary treasure!

When the Aldens spend the summer riding on a train, they get to live in a train car once again - this time, they're staying in Caboose No. 777. The number is hard to forget, and indeed, the children meet people at each stop who seem to know it well. It turns out Caboose No. 777 has an unusual past - one that comes with its very own mystery!

The Alden children, snowbound in a vacation cabin, discover adventure in surrounding wild life, three children they find in the snow, and a search for a lost recipe.

Because they do not want to be separated, four orphaned children run away and live in an old abandoned boxcar

When the Alden children accompany their grandfather to the Snow Haven Lodge, a series of odd occurrences threaten to disrupt the annual winter games

The Alden children suspect someone of trying to sabotage the local dog show

While staying on a Navajo reservation, the Aldens uncover some artifacts that may be evidence of an ancient village, artifacts that soon turn up missing

During a trip to New England, the Aldens learn about a mysterious ship, the "Flying Cloud," that had been lost at sea years earlier and that supposedly reappears during a storm

The Alden children suspect that the other guests are somehow involved when things begin to disappear where they are staying in Washington, D.C.