While running near Grayskull, Battle Cat becomes thirsty and stops at the well near the castle. He-Man draws up a bucket of water for the tiger, questioning aloud whether he should just get a mechanical vehicle, since those don't need to stop for water (jokingly), but hears a battle raging underneath the well.
Upon climbing down the rope he discovers a system of underground caverns, which lead to a secret entrance to the castle. After fighting various enemies, He-Man, along with Man-At-Arms and Stratos, save Teela and Skeletor disappears, defeated.
He-Man decides he won't let Skeletor gain entrance to Grayskull by way of the secret passage, so he strikes at the walls of the cave until they begin to collapse, whereupon they all escape to learn that Battle Cat sought help for He-Man after he went down the well and realize that a live animal is better than any machine form of transportation.
Scans courtesy of Black.