The mysterious dog showed up at Jax's door just when she needed an adventure. But adventure sometimes brings trouble -- and dangers that even a great dog can't help you escape. In the Catskill Mountains, mystery is waiting . Just when she needs it most, a little excitement comes to Jax's mountain. First, a beautiful, giant dog stops at her door. Even though he has no collar, the Tibetan mastiff doesn't act like a stray -- and he seems to want to stay with Jax. Then lights appear in the old, abandoned monastery up the mountain. The mastiff, who likes being called Mo-Mo, leads Jax to the mountaintop.

There she meets a boy her age, Yeshi, who has come all the way from Tibet with his teacher to open the abandoned building -- and to search for a long-lost statue, possibly hidden away in the monastery. But someone else is searching for the statue, too, and when Jax's adventure turns dangerous, she'll have to count on her new friend, and the mysterious dog that's found her, to get her back down the mountain safely.