Marlo is a so-called marine biologist/dolphin expert who let Ken do an internship with her about the captured dolphins, which turned out to be false since she is more of creating wealth out of their sale rather than helping them.
Isla finds Emerald in captivity, but leaves when Barbie, Skipper and Ken arrive at the facility. Stacie, Chelsea and the puppies also arrive, after Stacie saw Emerald from their cabin. Ken tells Barbie and Skipper about the myth of the Gemstone Dolphins, and says that Emerald is in a pool for sick or injured animals to be rehabilitated. Emerald looks fine, so Barbie thinks he got into the facility by mistake. Barbie tries to free Emerald, but Marlo stops her.
Ken asks Marlo if Emerald is a Gemstone Dolphin, but Marlo says that they are just a myth. Isla transforms her tail into legs and tries to release Emerald, but Barbie stops her. Isla says that Emerald is a Gemstone Dolphin. She tells Barbie not to tell Marlo about Gemstone Dolphins, and agrees to wait until a vet checks Emerald the following afternoon. Marlo calls the vet, Hugo, and asks him to come tomorrow morning instead of the afternoon. She tells Emerald that he is going to make her a lot of money.
Barbie, her sisters, Ken and Isla go to a buffet, and Skipper takes a group photo. The next morning, Isla and Barbie go swimming and one of the Gemstone Dolphins touches Isla's necklace, transforming her back into a mermaid. Isla tells Barbie to keep it a secret. Skipper and Stacie see Hugo arrive by helicopter, and go to tell Barbie so they can say bye to Emerald. Isla and Barbie wonder why they hear Emerald even though they're far from the cove, and Isla finds a cave with confusing tunnels that lead to Emerald.
Barbie's sisters overhear Marlo telling Hugo to transport Emerald to the mainland facility as soon as possible. Skipper realizes her photo of Isla is a photo of a mermaid, and she takes a photo of the logo on Hugo's helicopter. The Gemstone Dolphins find Emerald and Ken opens the gate for them. Marlo orders him to capture the dolphins. Marlo leaves, and Isla and Barbie find the dolphins. Ken tries to free them, but the code to open the gate has changed. Isla no longer trusts Barbie, and says she will free the dolphins by herself. Barbie's sisters arrive and Skipper says Hugo's logo is for a sideshow company.
After trying the code again, a new code is sent to Marlo's master device. Barbie tells Ken and the puppies to distract Marlo while Chelsea gives the master device to Stacie and Skipper. Skipper syncs the device to her tablet and downloads the new pass code. Barbie apologizes to Isla, and Isla agrees to help. Isla reveals to everyone that she is a mermaid, and she gets in the pool with the Gemstone Dolphins. Marlo arrives and says she reprogrammed the master device, so the new pass code fails. Isla and the dolphins hide in the tunnels, so Barbie says they escaped.
Marlo and Hugo go to his helicopter to find Isla and the dolphins. Isla gives Barbie a shell that emits a signal, and tells Barbie to go to the other end of the tunnels and guide them through. Marlo and Hugo see them escaping, and pursue them. Barbie and Isla part ways, and Barbie gets caught in Isla's place. Barbie tells Marlo that a real marine biologist would use a research center to help animals, not exploit them. Hugo leaves in the helicopter, and Isla leaves with the dolphins. She was in the end humbled by Barbie after she faked her mermaid form in order to protect her new friend, Isla from being sold. It is unknown what happened