Ash's Electrium Z-crystal is missing. Rotom, who is a big fan of an Alolan detective TV show, is prompted to try and find it with Pikachu as his assistant, getting a new video camera function in the course of the story. In the meantime, Ash and the other students are all desperate to keep Kiawe, the only other Z-ring wearer, from finding out about the missing Z-crystal out of fear of his anger. (Source: Wikipedia)
