Mystery of Metru Nui (Bionicle Adventures, #1)

The city of Metru Nui is divided into six parts. Each part has a guardian--a Toa--who watches over the Matoran who live there. However, something strange is happening: Matoran are vanishing and it's up to the Toa to find them and to discover what is causing the disappearances. Do the Toa have the skills to solve this mystery? Or will they finally meet their match?