Pokemon Movie 20: Kimi ni Kimeta!

The film's plot tells the story of how Satoshi and Pikachu came to know each other. Pikachu was not cooperative toward Satoshi, but Satoshi only wanted to be friends with Pikachu. On the day they set out from Masara Town, both of them saw a Ho-Oh flying and made a vow to someday go and meet it.

(Source: ANN, edited)