Ep. 62: Crisis at Chargestone Cave!

62

After earning the Quake Badge, Ash and co. decide to head to Mistralton City next. Along the way, however, Ash notices Pikachu has fallen ill, and soon learns that a Joltik was feeding off Pikachu's electricity, as well as Iris' Emolga and Cilan's Stunfisk (being part electric). They eventually find out that Team Rocket is behind this phenomenon. (Source: Wikipedia)