The X + Y Files - Under Sasquatch Mountain

At twelve years of age, Xander Bookman witnesses what he believes to be a UFO. When he confides in a friend his entire school is soon making fun of 'Captain Kook/Moonbeam Bookman' suggesting to Xander that the UFO came from Uranus. After the name calling turns into bullying and a second black eye Xander's protective mother pulls him from school and sends him to stay with his relatives for a while.
Xander and his cousin Yzzie are nothing alike but after a rocky start they discover some common ground to bond over. The two of them embark on an adventure to solve a local mystery, however, it quickly turns into something very different when Yzzie disappears. Xander will stop at nothing to find Yzzie, traversing a pitch-black labyrinth leading him deeper and deeper under Sasquatch Mountain during which he must overcome encounters with monsters both real and imagined. Eventually this brings Xander face to face with an entire family of Sasquatch who share a remarkable connection with Yzzie.