A family starts out on a one-day mountain climbing excursion that becomes very exciting. In fact, it becomes an adventure for the former "boxcar" family. They unearth some mysteries and have a fine time in the process.

The Alden children have a home with their grandfather now, but their adventures are just beginning! The first surprise comes when they learn they'll be spending the summer on their private island. A kind stranger who lives there is always happy to help the children out, but does this new friend have a secret?