Like other Pokémon Normajean had to transport, Pichu was curious and went searching on the deck of the ship. When the Tentacruel went on the ship (due to Team Rocket's disturbance), Pichu was shown very loyal and defended Normajean, using Thunderbolt on them. However, it got attacked by a Poison Jab and was poisoned.
Brock had to treat it, but did not know of a way, as its poison was strong and couldn't be healed very well. However, he used humidity, ice and electricity to treat it, then brewed a potion. Since Pichu did not want to drink it, Brock's Happiny evolved into a Chansey and used Softboiled to get the poison effect worn off.
After Nurse Joy came to the ship, Pichu and the other Pokémon were treated. They were grateful to Brock and thanked him, while he got the idea of becoming a Pokémon Doctor.