Like other Pokémon Normajean had to transport, Cleffa was curious and went searching on the deck of the ship. When the Tentacruel went on the ship (due to Team Rocket's disturbance), Cleffa was shown very loyal and defended Normajean, using Pound on them. However, it got attacked by a Poison Sting and was poisoned. Luckily, Brock used his knowledge and some goods from the cargo and the passengers to treat it.
After Nurse Joy came to the ship, Cleffa and the other Pokémon were treated. They were grateful to Brock and thanked him, while he got the idea of becoming a Pokémon Doctor.