Ash and hisfriends 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 was in IL077: The Fourth Round Rumble where it defeated Jeanette Fisher's Bellsprout in the fourth round of the Indigo League.
Ash returned Muk to his team for a brief period in The Mystery Menace, where it was used to find a large Bulbasaur that was residing in the sewers.