According to Bulbapedia, it is, in fact, Episode 1.
http://bulbapedia.bulbagarden.net/wiki/Episode_1He quickly starts to ride to Viridian City, with storm clouds above him and the tens of Spearow in close pursuit. Soon, it starts to rain. He rides the bike over a ledge, and it falls on its side, which knocks Ash and Pikachu off. Ash finds himself looking at his injured Pikachu lying on the ground, with the Spearow approaching and the storm growing worse. Desperate to save his Pokémon, he leaves Pikachu's Poké Ball next to it, and then stands before the Spearow, telling them to attack him in order to give Pikachu time to get inside its ball where it would be safe. However, just as the Spearow are about to dive on Ash, Pikachu leaps up and over Ash's shoulder. It jumps at the flock of Spearow. The lightning from the storm is drawn to Pikachu, highly raising its potential and strength by vast amounts, and it uses a massive electrical attack to take out the entire flock, as well as the girl's bike. Pikachu is drained of stamina from the effort.