 The Thornberrys are in Uganda s Impenetrable Forest to film the rare bird The Great Crested grebe. While Marianne and Nigel go off to film the girl s argue over the use of the tape player. In the ensuing argument Debbie releases the commvee s brake and it takes off downhill. It finally comes to rest on a muddy river bank where it is firmly stuck. The girls will have to work together to free it.