Mystery of the Dark World (いそげ悟空!五行山のなぞ, Isoge Goku! Gogyozan no Nazo) is the thirtieth episode of the Piccolo Jr. Saga and the one hundred fifty-second episode in the Dragon Ball series. The episode first aired on April 12, 1989.
SummaryGoku continues to wave the Bansho Fan, but it is not working. Fortuneteller Baba warns Goku that the fan would not work and he must go to the Magical Furnace in the Dark World and meet Tajoro. Goku and Chi-Chi journey and enter the Dark World. Goku tries to summon his flying Nimbus, but the Nimbus does not want to come anywhere near the mountain. They jump over a large, spear-filled gap on the road and get chased by bats. The bats almost knock Chi-Chi into the spears. Then, they chase them down the path and encounter soldiers.
As Goku and Chi-Chi go further, things become more dangerous, as they encounter more monsters, but they discover all the dangers are imaginary, so Goku jumps into a dragon-monster's mouth and they successfully cross over and proceed.
Back at the castle on Fire Mountain, Ox-King gets trapped at the peak of the castle. Goku and Chi-Chi arrive at the furnace. They meet Grandpa Gohan at the furnace, Goku introduces Chi-Chi and a mini reunion occurs. Goku assumes that Gohan is Tajoro, but Gohan tells him Tajoro is actually called Annin, and he is stationed at the furnace to guard her. Goku wants to put out the furnace, which Gohan and Annin object to. Gohan and Annin would not let him put out the furnace, because if they do, the plains would be thrown into chaos.