The Mattigan kids don’t believe in things that go bump in the night. After all, their dad is famous for proving such things are impossible. But when their long-lost Grandpa Joe shows up with his Mysterious Monster journal begging for help, the siblings find themselves drawn into a search for Bigfoot. Along the way, they’ll have to deal with meddling babysitters, suspicious psychics, a youtube disaster, and their furious father. To solve this mystery, Maddie, Max, and Theo will have to rethink what’s possible — and make lots of peanut butter and banana sandwiches.