Yung was once a student at the Pokemon Institute, but he was expelled because his experiments were considered cruel to the Pokemon. This left him bitter, and he wanted revenge.
Yung invented the Mirage Pokemon system, creating powerful Mirage Pokemon who can use any attack, easily beating Misty in a Pokemon battle. Before Ash could battle Dr. Yung, he and Professor Oak was captured by the Mirage Master.
Dr. Yung was in fact the Mirage Master, as he was using a hologram so both could be seen at the same time. As the Mirage Master, he tortured Professor Oak to gain the password to Oak's laboratory database. Although Oak resisted the torture, Mirage Master had his Mirage Pokemon capture Pikachu and attempted to scan its memories for data, causing it great pain and forcing Oak to give up the password. With this access and his ability to hack Pokemon centers, Dr. Yung began forming perfect Pokemon free of any flaws which he despised.
He completed his perfect Pokemon, a Mirage Mewtwo, just as Ash, and later Misty and May, arrived to confront him. He attempted to kill Professor Oak, who defended himself with a Dragonite, and Mirage Master deployed missiles to create a larger area for his Mirage Pokemon so Mirage Mewtwo could be unleashed upon the world. He created a Mirage Entei, Zapdos, and Articuno to fight alongide Mirage Mewtwo for this.
Professor Oak eventually deduced Mirage Master's true identity as Dr. Yung, who finally reveals himself to the heroes. Yung had become annoyed at a Mirage Mew appearing, whom he described as a flawed specimen. He then orders Mirage Mewtwo to battle Ash and the gang, and nearly killed Pikachu before Mirage Mew entered the battle to stop Mirage Mewtwo, and Pikachu used Volt Tackle to destroy Mirage Mewtwo and the entire Mirage System, setting his lab on fire. Unfortunately, this destroyed Mirage Mew as well.
With Dr. Yung's plans ruined, Professor Oak demands him to surrender himself, but Yung refuses, saying that he still has much more work to do, and instead walks into his burning lab, which later crumbled down to pieces. Yung was never seen again following this, and was not yet found by police or firefighters during the ending scene. It is presumed that he was crumbled to death.