The attraction is located in the Marine Life Institute at the heart of Port Discovery. The story goes where a cast member explains to the guests how difficult it was to explore the oceans in the past, specifically the marine life and behavior of fish. Scuba divers, submersibles with claws, and even subs designed to look like fish would all scare the fish away. One day, the brilliant scientists of the MLI came up with the idea to make a submarine that was not only the same shape of a fish but the same size as a fish and thus they developed a special material called Chiziminium, named after the Japanese pun of saying 'Shrink-amin-iam' and they created a whole fish submarine built with Chiziminium, that shrinks with the guests inside once electricity is applied as the cast member demonstrates before the people get on the ride.
The amazing fish sub is called the SeaRider, a pink and purple fish with big blue eyes and a smile on its face, built with artificial intelligence and a personality similar to that of a regular fish, allowing other real fish to accept the sub as an actual fish. The guests experience the whole ride not as a person within a shrunk sub but as a brand new character within the Finding Nemo/Dory universe named Sheila by Dory.
Once the Searider shrinks, we either arrive during a hide and seek game or a field trip with Mr. Ray and his class. Either way, guests are invited to join in the fun and within that time Dory reads the name of the sub and calls her new friend Sheila, as do Nemo, Marlin, and the others. When guests arrive at the drop off, we either learn that Marlin followed Mr. Ray to keep an eye on Nemo and Dory or we meet Hank and Dory introduces Sheila to the others.
At that moment, either Squirt or Crush emerges from the current and the fish swim into the current to interact with the turtle. Crush helps steady Marlin when he almost shoots out of the current and Squirt playfully headbutts Sheila and comments that her head is as hard as a rock before we go on a wild ride across the current before we exit out.
When guests exit out, they either end up surrounded by Jellyfish or end up in a kelp forest with Otters. In the Jellyfish scene, Dory plays with a baby jellyfish before it stings her and Marlin flicks it away. Soon after a full grown jellyfish stings Sheila, Marlin saves her before they bounce across the Jellyfish forest with Nemo and Dory and make it out as Marlin protects Sheila from being stunned again. In the Kelp Forest scene, two playful otters swim around the kelp forest with Nemo, Dory, Marlin, and Sheila playing tag with their new friends before leaving after they nuzzle guests with the other fish.
After that, guests end up in a kiddie touch pool or at the underwater containers' site. At the Touch Pool scene, guests meet Hank outside a pipe where Nemo accidentally gets sucked into the pipe and Marlin follows, so as Dory, Sheila and Hank. Once they reunite, a human hand scares them and the group swims across the touch pool before Sheila is grabbed and the others look on in horror as Sheila is held eye level with Darla (a character who did not physically appear in the sequel) who at first is happy she grabbed a fish. Darla, however, begins to suspect that Sheila is not a real fish before Hank places a tentacle on her face, scaring Darla as she flings Sheila into the air before Hank saves her by catching her within a glass cup which already has Marlin, Dory, and Nemo inside and he moves past a terrified crowd of guests before they manage to get over the rails and escape into the water where Hank embraces them with a hug, which ends briefly due to her being embarrassed.
In the Midnight Zone scene, Marlin asks Dory to call for help and she calls out for Bailey and Destiny via whale talk, making the hermit crabs shush her before the giant squid emerges from the container and chases the fish across the area before trying to devour Marlin, Dory, Nemo and Sheila. The beast manages to corner them and attempts to eat them with one bite once they are pulled into the creature's beak. However, because of Sheila's super tough metal submarine body, the squid's beak cannot break through her body or eat her, sparing her and saving Nemo, Marlin and Dory in the process. This buys Bailey and Destiny enough time to arrive and save their small friends before they knock the creature into a container and it falls into the darkness as they all cheer and Dory brings her friends in for a group hug.
From there, guests are either flown across the sea by Becky and the loons that carry guests by their feet across some rocks and sea lions before we are dropped into the water or we follow the stingray migration all the way back to the reef while Marlin sings along. Once guests arrive back home, everyone gathers around to bid Sheila a farewell and at that moment, Sheila returns to her normal size, startling whomever is left behind saying extra good byes for their friend, whether it's Dory, Hank, Pearl, Peach, Bloat, or Dory's parents Jenny and Charlie. The ride ends with a cast member saying goodbye and to come again.