The series is loosely connected to the Yu-Gi-Oh! Duel Monsters series (known internationally as simply Yu-Gi-Oh!), which was made by Nihon Ad Systems and aired on TV Tokyo; both anime series are based on the same manga series. Whilst the NAS version bases its story from Volume 8 of the manga onwards, focusing on the Duel Monsters trading card game, the Toei anime is adapted from the early volumes, telling the origins of Yugi's Millennium Puzzle and the Shadow Games that he plays.
The 1st series, which aired on TV Asahi from April 4, 1998 to October 10, 1998 for a 27-episode run, was produced by Toei Animation. Manga plots were re-written so that they could take up an entire episode time. Also, the level of violence was reduced between the manga and the anime. Finally, a minor character in the manga, Miho Nosaka, became a main character in the first series along with Yugi Mutou, Katsuya Jonouchi, Anzu Mazaki, and Hiroto Honda. Characterization was slightly modified. For instance, Miho is a "ditz" in the anime while in the manga she is a shy student librarian.
New characters and plots, such as those relating to the four game masters, were added, and there are more occurrences of Seto Kaiba and Duel Monsters (Magic and Wizards in the original Japanese manga) than in the corresponding manga, and in some cases, plot points and elements were taken from volume 8 onwards of the manga, for instance field advantages as seen in Duelist Kingdom and Jonouchi's sister Shizuka, who was introduced later in the manga. Unlike the second series, the card game (Duel Monsters) is not the sole focus of the show. The last episodes of the series focus on the battle between Yugi and Dark Bakura. This series had different voice actors and different character designs than the second series (e.g. Seto Kaiba's hair is green in Toei's anime). The series has not been licensed for an English language release.
The 1999 Yu-Gi-Oh! film uses the characters from this anime series.
Yugi Mutou is a young boy who solves the Millennium Puzzle, wishing for true friends. While he managed to get some friends—Katsuya Jonouchi, Hiroto Honda and Anzu Mazaki—he also gains a mysterious host. Whenever someone hurts or threatens one of Yugi's friends, Dark Yugi takes over Yugi's body and challenges them to a shadow game which vary, and relate to the opponent whom Dark Yugi is facing.
One recurring story arc in the series involves a boy named Seto Kaiba, world champion of the Duel Monsters trading card game, who attempts to steal a rare card that Yugi's grandfather possesses. After obtaining it through dirty means, Dark Yugi challenges him to a Shadow Game, Kaiba cheats in the duel, but after some strategic moves made by Yugi, the duel ends in a draw when Yugi uses Blue-Eyes White Dragon, given to him by his grandfather. Obsessed with defeating Yugi to determine who is stronger, Kaiba recruits four gaming experts, the "Game Masters," throughout the series to try and defeat Yugi and test the limits of his strength, but they are each defeated. Finally, Yugi and his friends are challenged by Kaiba and his younger brother Mokuba to take part in a series of life-or-death games called Death-T, after which Dark Yugi and Kaiba face off once more; the match ends in Yugi's favor, who finally gives Kaiba his penalty and purges the evil from his corrupted heart.
Another story arc involves Ryo Bakura and his Millennium Ring, which is similar to Yugi's Millennium Puzzle; however, the Ring contains a malevolent Dark Bakura. At the end of the series, Dark Bakura challenges Yugi and his friends to a board game/role-playing game called Monster World in which their souls are sealed within their own character pieces. However, with Dark Yugi controlling Yugi's body, the five friends are able to defeat Dark Bakura and save the real Bakura.
- Yugi Muto Voiced by: Megumi Ogata
- The series' main character, a timid boy who likes to play games. One day, he solves the Millenium Puzzle and inhabits the spirit residing it. Whenever someone cruelly mistreats his friends, this 'Other Yugi' takes over and challenges them to a Shadow Game, life threatening games that reveal a person's true nature. For those who fail his Shadow Games, Dark Yugi punishes them with a penalty game.
- Katsuya Jonouchi Voiced by: Toshiyuki Morikawa
- Yugi's friend. He used to pick on Yugi for always playing games by himself, but after Yugi risks himself to protect him from a bigger bully, Jounochi befriends him.
- Hiroto Honda Voiced by: Ryotaro Okiayu
- Yugi and Joey's friend, who is head of the beautician department (or simply the janitor.) He has a crush on Miho Nosaka, but can never seem to win her heart.
- Anzu Mazaki Voiced by: Yumi Kakazu
- Another one of Yugi's friends. After being first saved by Dark Yugi, she becomes interested in this side of Yugi and will occasionally put herself in danger in order to bring him out.
- Miho Nosaka Voiced by: Yukana Nogami
- A one-shot character in the manga, she is more featured in this series. Although Honda has a crush on her, she herself seems to get a crush on Bakura.
- Seto Kaiba Voiced by: Hikaru Midorikawa
- President of Kaiba Corporation, a producer of games. After first facing Yugi in a Shadow Game of Duel Monsters, in which he brought the game's monsters to life, Kaiba becomes inspired to build a holigraphic system to replicate that feel. He constructs the deadly theme park, Death-T, to test Yugi and his friends. Unlike other media in which he is depicted, Seto is shown with green coloured hair in this adaptation.
- Mokuba Kaiba Voiced by: Katsue Miwa
- Seto's younger brother, who is an expert at Capsule Monsters Chess (though will often rig games in his favour).
- Ryo Bakura Voiced by: Tsutomu Kashiwakura
- A transfer student with an interest in role playing games. He wears the Millenium Ring, which also possesses a Dark Spirit, who has the power to trap people's souls into Bakura's games.
- Shadi Voiced by: Kaneto Shiozawa
- An egyptian man who possesses the Millenium Key, used to see into people's minds, and the Millenium Scales, which weighs a person's sins. When he becomes aware of Yugi's possession of the Millenium Puzzle, Shadi puts Yugi's other self through a series of tests to see if he is worthy.
Kazuki Takahashi, the author of the original Yu-Gi-Oh! comic book series, said that he was "half excited and half scared" when he received the first offer to have Yu-Gi-Oh! animated. Takahashi felt this way because this meant allowing someone else to handle a property that meant so much to him. He wondered if the themes in the original manga would translate properly to the television series as he headed to an anime post-recording session. According to Takahashi, after he arrived he felt relieved when he heard the voices of the seiyū. Takahashi commented "these pros not only gave the characters a voice, they gave them life! I was moved!"
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- Yûgiô at the Big Cartoon DataBase
- Yûgiô at the Internet Movie Database
- (Japanese) Toei Yu-Gi-Oh! first series page
- (Japanese) Toei Yu-Gi-Oh! first series property information page