Akira Toriyama is most famous for his 42-volume manga Dragon Ball, but that is hardly the only thing manga he's ever produced. Toriyama was also the creative force behind Dr. Slump, a gag manga that ran from 1980 to 1984. Just a few months after Dr. Slump's completion in September of 1984, Toriyama would start work on Dragon Ball. In many respects, Dragon Ball followed Dr. Slump in terms of style. Both were comedic series focused around incredibly powerful children in a world full of science and magic, and both series followed a whimsical set of characters in stories filled with gags and puns.
While it might seem odd considering the action and planetary destruction that Dragon Ball would eventually become known for, Dragon Ball and Dr. Slump felt as though they had been cut from the same comedic cloth in thh=eir earlier years. This is why it made total sense for the two to crossover, which would create an interconnected universe of Toriyama manga that started when Arale of Dr. Slump met Goku and has only grown since then.
Who is Arale?
Arale Norimaki is the main character of Dr. Slump. However, much like the monster in Mary Shelly's Frankenstein, she's the creation of the book's titular scientist. Arale is a super-powerful child android created by Senbei Norimaki, who's nicknamed Dr. Slump due to how he constantly seems to be in a slump. She's powerful and super strong but needs glasses to see and is very naive. She often gets herself into ridiculous misadventures. But due to her incredible powers, she manages to get out just fine.
The series introduces Senbei as a failed inventor, whose inventions are often extraordinary but flawed in some way. Later on, the manga introduces a villain named Dr. Mashirito, who apparently is based on Toriyama's real-life editor. Still, even though this villain is a major antagonist, the series isn't about action. He's more comparable to the goofy, mostly fun Pilaf than any of Dragon Ball's other more serious villains.
When Dr. Slump ended, Toriyama started right away on Dragon Ball, which, at the time, bore a great deal of similarity to Dr. Slump. It seemed inevitable that Goku would eventually take a trip to Penguin Village, where Dr. Slump took place.
Dr. Slump References
There are numerous references to Dr. Slump throughout Dragon Ball -- especially early on. Various characters make short cameos throughout the series. Arale appears on TV and posters in Bulma's capsule home multiple times in the early series, and other characters can be seen as faces in crowd shots.
King Yemma is one of Dragon Ball's odder references to Dr. Slump. Yemma first appears in Dr. Slump in generally the same role as lord of the other world. However, the King Yemma who appears in Dragon Ball during the Saiyan Saga looks completely different, even though the worlds seemingly share a continuity.
Furthermore, when Dr. Slump returned briefly in the mid-90s as The Brief Return of Dr. Slump, the manga included brief cameos from Goku, Pan, Trunks, and Giru (all in their Dragon Ball GT appearance) in the Grand Tour Spaceship. This is the biggest -- possibly only -- canon reference to GT in the manga continuity.
Arale has also appeared in multiple Dragon Ball Z video games. Most notably, Dragon Ball Budokai Tenkaichi 3, which featured Arale as a playable character.
The First Crossover
The first of two major crossovers happened early on in Dragon Ball in the General Blue Saga -- a subset of the Red Ribbon Army Saga, one of the first big arcs in Dragon Ball. As Goku collects the Dragon Balls to stop the Red Ribbon Army, he travels to Penguin Village, where he encounters Arale. It's clear that this crossover takes place slightly before the end of Dr. Slump, as evidenced by the fact that Arale has yet to become mayor of Penguin Village.
Arale proves to be even stronger than Goku at this point, throwing herself at the Red Ribbon soldiers with childlike zeal. This event motivates Goku to train even more in order to surpass Arale in terms of strength. This moment was adapted again in the 1997 remake anime of Dr. Slump.
Years later in Dragon Ball Super, however, Goku would meet Arale yet again in a brief crossover in Episode 69. This is where things become a little odd because it manages to apply the gag reality of Dr. Slump to the more structured world of Dragon Ball Super. The reoccurring Dr. Slump villain, Dr. Mashirito, appears in Super as a ghost who has escaped from Hell and is causing all sorts of discord and chaos. This results in Goku and Vegeta being forced to fight Arale, who, despite last time being only stronger than Kid Goku, is now on par with Super Saiyan Blue.
Of course, the odd thing here is that, despite 30 years of canon passing between appearances, Arale, Senbei, and the rest of the characters all look even younger than the last time they appeared. Then again, considering that Vegeta multiple times refers to Arale as a "gag character" in this series, perhaps we as the viewers aren't meant to take this seriously.
While Dr. Slump is the most noteworthy Toriyama anime to crossover with Dragon Ball, it isn't the only one. Ignoring brief special crossovers, like Cross Epoch where Goku and friends meet the Straw Hat Pirates from One Piece, the continuities of other projects Toriyama has worked on have truly been woven into the continuity of Dragon Ball.
The most notable example of this is Jaco the Galactic Patrolman, a gag manga by Akira Toriyama that ties into Dragon Ball by the manga's conclusion. Since then, it has become further woven into the canon of Dragon Ball Super, starting with the Resurrection F arc.
However, before that, there was the slightly lesser-known manga Nekomajin, a Dragon Ball parody series that featured multiple Dragon Ball characters. Throughout eight one-shot chapters released sporadically between 1999 and 2005, the series features its titular Neko Majin Z, a strange cat creature who exists in the short aftermath of the Buu Saga. He ends up meeting lost Saiyans, Frieza's son Kuriza and Vegeta, and he claims Goku as his mentor and master.
Due to existing in the nebulous post-Buu Saga Time Skip -- even further beyond Super's current timeline -- it's unknown if Nekomajin will ever tie into Dragon Ball Super. Though, interestingly enough, Vegeta's line about never fighting gag characters in the Dr. Slump/Dragon Ball Super crossover is lifted from Vegeta's appearance in Nekomajin.
While all of these Toriyama characters still exist together in a loosely connected universe, they're all defined, to some extent, by his irreverent sense of humor, which keeps the Toriyama voice from taking itself too seriously.
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