Set in Japan during the 21st century, the underground is inhabited by blue anthropomorphic mole creatures called "Undergrounders". Their main source of food is colored blocks, which are generated from their factories across the world. One day, during a normal day, their blocks overflow various cities across the globe. With the overworld in a panic, they summon their most prestigious driller: a cute energetic boy named Susumu Hori, to deal with the situation.

Susumu then drills down these blocks, while avoiding being crushed and wasting all of his air supply. When he reaches the bottom, he tells the undergrounders that their blocks are overflowing earth. The king states that they are stacking up in case of an emergency. Susumu bribes the undergrounders to stop producing blocks and warns them next time.

Then one day later, Susumu, Puchi, Anna Hottenmeyer, Juno, Ataru Hori, Usagi, Taizo Hori, Masuyo Tobi and Holinger-Z go on adventures to stop Dr. Manhole and the Ankoku Drillers as well.

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