From Bulbapedia, the community-driven Pokémon encyclopedia.
Days passed and mother Kangaskhan had some difficulty feeding her two children. However, she always showed a positive attitude towards them.
A few more days passed and after the baby walked away from his mother running after a wild Mew he was playing with. Not before, Pichu and his
"mother " go in search of the little Pokémon. During a rainy day, Kangaskhan and his family take shelter in a cave to protect themselves. Seeing his mother's suffering to take care of him, Pichu decides to separate himself from the Pokémon that welcomed him when his mother eventually falls asleep.
Amid the sadness of separation, Pichu finally evolves into Pikachu and fares farewell to his family.