The Secret Decoder Ring (S04E07)

After Billy discovers a "hidden decoder ring" in his box of Blasto Bits cereal, he receives a holographic transmission from an unusual creature, hinting at the "Key to Existence" hidden in a deserted mine shaft. Eager to uncover this secret, all the kids in Endsville, equipped with their own decoder rings, dash to the mine. However, they soon find out that it’s a clever trap set by Lubbermouth, a worm-like being from Grim's history, who seeks to escape her role as the guardian of the secret. When Mindy peers into the abyss, she unwittingly becomes the new protector and is bound to remain there forever.