While patrolling the planet, Astro Boy comes across a Satanic ritual and gets an idea. Later in the night, Astro Boy gathers all the items seen in the ritual and chants the phrase "by the powers of the dark lord, bring to me the door to the underworld". Dr. Tenma, who had just walked in, questions Astro Boy on his actions. Astro Boy explains that he had earlier overheard people telling Dr. Tenma that his son Toby was burning in Hell, and was planning on going to Hell to get him back.
Dr. Tenma decides that Astro Boy won't be going to Hell, not alone. With the portal opened, the father/adoptive son duo embark on their quest to find Toby and bring him back to the world of the living. After falling through a 40-mile cavern, the duo find themselves before a massive gate, which is guarded by a demonic-looking dragon, which Astro Boy manages to defeat. He then smashes his way through the gate followed by Dr. Tenma.
The duo then travel through Hell, battling their way through various demons and damned souls along the way. Eventually, they come across Eric Bloodaxe the Viking, who agrees to aid them on their quest. Eventually, they come across a large palace, which Eric reveals to belong to Satan himself. Astro Boy breaks the door down and searches the palace for Satan. Eventually he finds him in a room arm wrestling with God. The two beings, both angry about having their game interrupted, and both feeling threatened by the existence of Astro Boy, team up to take him on.
Dr. Tenma and Eric Bloodaxe join the fight, and an all-out battle wages between the two teams, and eventually, Astro Boy defeats both beings and frees Toby from a cage in the room where God and Satan were arm wrestling. With Toby free, Astro Boy, Dr. Tenma, Eric Bloodaxe, and Toby return to the world of the living to rejoice and celebrate.