The day starts when the Octonauts watch Dashi taking pictures of a strange underwater cave until Shellington discovers that it's a whale shark's mouth. Dashi gets trapped inside the whale shark as later on the whale shark swallows Captain Barnacles and Kwazii as well while Peso from the outside tries to help the whale shark with a tummyache. Barnacles and Kwazii discover Dashi in the stomach, and they get out to walk toward the mouth, which is closed shut. They were discovering things. In the UK series, this is the very first episode that premiered the show of the Octonauts.
Captain Barnacles, Shellington, and Peso are looking at pictures being sent to the Octopod by Dashi, who is exploring an underwater cave. Peso notices the cave floor is red, and wiggly. Barnacles comments on how some rocks hanging from the cave roof look "kind of familiar". Shellington then notices that they are not rocks, but teeth, and that the floor is a tongue! Barnacles and Shellington then realise the truth - Dashi's not inside a cave, but a whale shark! The Captain contacts Dashi and tells her to leave the "cave" immediately, and she swims towards the mouth, but drops her camera and has to swim back for it. The whale shark closes its mouth and she therefore does not make it out in time. Shellington can't radio her as the signal has been lost now the mouth is shut. Captain Barnacles tells Peso to sound the Octo-Alert! He commands all the Octonauts to report to the launch bay.
Captain Barnacles informs the other Octonauts (Kwazii, Tweak, Tunip and Professor Inkling) that Dashi has been swallowed by a whale shark. Tunip asks Shellington if a whale shark is a whale. Shellington tells him that a whale shark is not a whale, but a shark (which is a fish) and the biggest fish in the ocean. Professor Inkling mentions how the fish is a filter feeder, which means it just opens its mouth and swallows whatever's there (meaning it didn't swallow Dashi on purpose). Barnacles instructs Kwazii and Peso to enter the GUP-A to go and save her. Tweak gives the captain a whale shark detector so they'll know where the whale shark is. She opens the Octo-Hatch and they go.
Peso keeps a lookout in the GUP-A while Barnacles and Kwazii plan tactics outside. Unfortunately, despite Peso's attempts to warn them, they do not notice the whale shark approaching and are also swallowed up...
Inside the whale shark
Barnacles and Kwazii are now inside the whale shark with Dashi. However, she doesn't seem to be anywhere around, so a little exploring is in order. Kwazii notices a hair clip belonging to Dashi, on the floor on their side of the shark's sponge wall, which means Dashi is on the other side. They squeeze through and fall down a dark tunnel! They hear a strange clicking sound, and wonder what it is. Then out of nowhere, Dashi appears clicking her camera! She tells them they're inside the whale shark's stomach! The Captain says they need to get out now, before they sink through. So they climb back up the tunnel and through the sponge wall, all the way back to the mouth.
Peso and the whale shark
Outside, Peso is trying to contact Captain Barnacles and Kwazii, with no success, when the whale shark moans in pain. Peso, being a medic, goes to investigate. He check's the whale shark's ear and nose, but then sees the pain is in the whale shark's stomach. However, the whale shark seems unwilling to open its mouth. Peso reads his medical book and finds out, if you tickle a whale shark's gills, he opens his mouth! So that's exactly what he does.
Barnacles, Kwazii and Dashi escape
With the combined effort of the three Octonauts inside trying to push open the mouth, and Peso tickling the gills, Captain Barnacles, Kwazii and Dashi escape from the whale shark! Its stomachache is now cured, and the Octonauts are free, but Kwazii later makes the mistake of calling it little, causing it to burp at him (and inadvertadly the others) in retaliation.
Shellington and Dashi are deciding which of the pictures Dashi took to send to National Seographic (a play on National Geographic). Professor Inkling says they should send them all, when a picture appears on the screen of Barnacles and Kwazii looking terrified in the whale shark's stomach. They quickly say she should probably skip some pictures. Tunip and another Vegimal then imitate the picture on the screen, making everyone laugh, and the episode ends.