When Timothy's mother is dying from a terrible plague stalking the Kingdom he doesn't know what to do. Then his best friend and King's Champion Tree the Cave Troll takes him to see the Wiseman and he learns of a magical stone called the Moonstone which can cure any illness. The only problem is the stone is sacred to Werecreatures who guard it on the fastness on the Isle of Elronth. These creatures - who can change shape into animals - will stop at nothing to stop the stone falling into anyone else's hands, changing those who attempt such an outrage into werecreatures themselves using the magic of the stone. But with the plague devastating both his family and nation, there is only one thing that Timothy and Tree can do, risk being turned into monsters by stealing the Moonstone from right under the snouts and jaws of the Shapeshifters...