Twelve-year-old Jack, his younger sister, and the family's teenage foster child Troy go to Yellowstone National Park, where Jack's mother, a wildlife veterinarian, is investigating the report that wolves reintroduced to the park have killed a dog there.

When a cougar attacks a hiker in Mesa Verde National Park, the Landons have another mystery on their hands. Jack and Ashley are used to visiting parks with their wildlife veterinarian mother and photographer father, but this time their hands are full with a foster child called Lucky Deal. As Lucky and Jack become friends, Ashley becomes suspicious: What did Lucky take from the burn site? Who did she call in the night? In a midnight showdown at

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The Landon family travels to Glacier National Park to investigate why grizzly bear cubs are disappearing and become involved with a ten-year-old Mexican runaway boy.

While she studies condors in the Grand Canyon a scientist's life is threatened, and the strange, hostile, teenage computer whiz in her family's foster care might be involved.

While at St. John National Park in the Virgin Islands for a seminar on coral reefs, the Landons help a wealthy thirteen-year-old to find his birth mother, who he believes is in danger.

While lost in a tunnel at Carlsbad Cavern, thirteen-year-old Jack, eleven-year-old Ashley, and their eight-year-old foster brother, Sam, think bats and darkness are their worst problems, until they stumble across thieves.

While their mother investigates a series of bear attacks in and near Gatlinburg, Tennessee, Jack and Ashley learn about country music and Cherokee people from two new friends, one of whom is keeping a secret.