"Brush your teeth, or the Cavity Express will come get ya!" Dentist apprentice boy EGG, and his side-kick cavity pal C1 hop into the mechanical-biological-train "Cavity Express" to explore through cavity world to accomplish necessary training needed to achieve the coveted DOCTOR title. Toothbrush and Toothpaste as his arsenal, Boy Egg resists against bad-nasty-cavities to rush for the "End of the Journey Station".