Ash and his friends were traveling through Gringey City, which was in the middle of a blackout. The reason for this is due to the power plant being over run by a gang of Grimer. After the Grimers were defeated, the gang's leader, a Muk, challenged Ash who used Pikachu and a wild Magnemite who had been following Pikachu do to an electrical build-up. After Muk was weakened by Magnemite and Magneton, Ash managed to catch it. However, since its smell could be smelt even through the Poké Ball, Ash sent Muk straight to Professor Oak.
During his stay at the lab, Muk developed an attachment to Prof. Oak and would constantly smother him whenever Ash would make a call to the lab. Muk has been used in only two major battles since it was captured; the first of which when it defeated Jeanette Fisher's Bellsprout in the fourth round of the Indigo League.
During the Silver Conference, Muk battled against Gary. It actually caused probems for Gary's Powerful Blastoise, as it was able to endure all of his attacks with ease, but it was a stalemate, as even it's strongest attack Sludge Bomb was deflected by Rapid Spin. Muk was also able to tank all of his Scizor's attacks as well, but it was thrown into the air and was hit with Swift, which was a 1 hit KO.
Ash called Professor Oak to inform him of his loss at the Vetress Conference. However, Muk smothered him in mid-call. Ash was happy to see one of his old friends, but Professor Oak was not happy being crushed.
In SM042, Muk was released by Brock to being compared with an Alolan Muk which it was released by Misty. Upon seeing Ash, Muk slumps him down in affection after he reunites with it.
|Body Slam||The Fourth Round Rumble|
|Poison Gas||A Tent Situation|
|Sludge Bomb||Can't Beat the Heat!|
|+ indicates this Pokémon used this move recently.*|
- indicates this Pokémon normally can't use this move.
- Ash's Muk has used less moves than any of Ash's other Pokémon, having only used three attacks on screen.
- Ash's Muk has only one on-screen victory so far, against a Bellsprout.
- Muk was Ash's first non-Unova Pokémon (excluding Pikachu) to appear in the Best Wishes Series.
- Muk is Ash's first Pokémon, aside from Pikachu, to appear at least once in every series.
- Muk is Ash's only pure Poison-type Pokémon.