Psyduck goes missing. Marina returns Psyduck. Marina and Misty have a Pokémon battle. A wild Golduck is mistaken for Misty's Psyduck. Team Rocket tries to catch Pikachu and Golduck. Misty and Marina finish their battle Psyduck comes out of it's Pokéball, Golduck runs off.
Ash and Co. Arrive at Kinnow Island and meet a girl named Marina, who was just as crazy about Water Pokémon as much as Misty. They noticed that Misty's Psyduck's tail had begun to glow, meaning it was a sure sign it was ready to evolve. During a battle between Misty and Marina a Golduck Appears and Misty thinks her Psyduck evolved, but after the battle with Team Rocket, when Misty called Golduck back her Psyduck popped out, revealing that it never evolved so poor Misty is stuck with Psyduck.
Pokémon Facts from this episode
"Golduck is very adapt at using it's webbed hands and feet, making it the fastest swimming Pokémon of all." - Dexter
This was the episode that premiered worldwide the special Mini-Misty Lure.
Brock was mentioned by Misty in this episode because the Golduck she thought was her Psyduck evolved liked to flirt with girls, but unlike Brock, the Golduck is successful at flirting.
The "Who's that Pokémon?" in this episode is Mewtwo, wtf?