The gang visits the Grand Canyon to go rock hunting, where they are caught in the middle of an earthquake. Once the earthquake subsides, the gang witnesses a Tyrannosaurus Rex, and the gang decides to follow him. The dinosaur crosses a bridge on the canyon, and he destroys the bridge once he crosses, causing the gang to fall into the canyon, but Fangface uses the tent as a parachute to drift them to safety. The gang goes to the canyon institute to report on the dinosaur, but the scientists are skeptical. One scientist, however, thinks the earthquake might have released the dinosaur from a previously sealed underground cave, and the only way to prove the dinosaur exists is to visit Dinosaur Cavern. Fangface and Puggsy find the T-Rex, and Fangface tries to take him on all by himself. The ensuing pole-vaulting escape attempt is almost foiled by a mysterious lasso-wielding man, but they escape just in time. Now the T-Rex is on a direct path for the city, and the gang has to find a way to s