After Marlin survives the Jelly attack, he is picked up by turtles and taken on the East Australian Current to get them to Sydney. As he tells the story of his journey so far, the baby turtles pass it on to other fish, who tell bigger fish, who tell crabs and so on until it reaches Nigel the pelican.
In the Dentist's aquarium, Nemo tries to escape before he is going to be taken home by the evil Darla, but he almost dies in the filter.
The pelican shows up and tells Nemo that his dad is searching the ocean for him. Inspired, Nemo blocks the filter so that the tank fills up with dirt - and when cleaned by the dentist he'll make his escape.
Marlin and Dory are eaten by a whale, and Marlin loses faith until it turns out Dory really can speak whale - and they are spouted out into Sydney Harbour.
In the aquarium the Dentist has installed a new super infra-red system that has already cleaned the tank - and then his little niece Darla arrives. The fish panic.
A pelican grabs Marlin and Dory, but Marlin makes him spit them out. The friendly pelican then picks them up to rescue them from all the seagulls.
Nemo plays dead and is about to be binned, but Nigel flies in to distract the dentist. Gill (Willem Dafoe) hurls himself out the tank using the volcano device and lands on Darla's head to stop her shaking Nemo. Nemo is then flushed down the sink, and the aquarium's leader is dropped back into the tank.
Marlin thinks Nemo is dead as he saw his son lifeless in the bag, and is dropped off back at the ocean. He leaves Dory as he wants to be alone. Meanwhile Nemo escapes from a sewage pipe and finds Dory - and together they find his old man -- but just as a big net traps them all. Nemo has a plan - for all the fish to swim down, which breaks the ship's pulley system.
They all get back home safely, with Marlin a changed fish and their father & son relationship as an example to other ... fish.
As it ends, we see that the heroic aquarium fish have clogged up the tank again and, hopping around in bags while out the water, have escaped to the ocean.