When their friend Rex goes missing, Dac and Steg set out to find him. They search his favorite hideouts, from a dark cave to a steep canyon. But he's nowhere to be found. They ask everyone they meet if they know where Rex might be. Someone saw him at the waterfall. Someone else saw him at the bridge. But where is he now? Rex is seriously stuck in a pit of mud and muck. Can his friends come together and figure out a way to free him, or will they end up stuck in the muck, too?