Pablo and Tyrone are deep-sea explorers who plan to discover a mermaid. They find two, who do everything to get rid of them, because they think they plan to ruin their garden.
Pablo and Tyrone are in the backyard wearing red winter caps. They introduce themselves as Commander Pablo and Navigator Tyrone, undersea explorers. Tyrone finds the slide in the backyard, and Pablo brings out a camera and gets the "undersea playground slide" on film. Tyrone tells him to come down, and Pablo does so. Tyrone asks Pablo when they will start the most exciting moment of their explorer lives: the day where they discover a mermaid. Pablo answers by telling him to get into their submarine while singing Into The Deep. The backyard transforms into the undersea coral reef.
Pablo and Tyrone are shown in their explorer caps inside the submarine used to bring them through the deep. They sing Into The Deep again while navigating deeper into the ocean. They have reached many miles below the surface, and they approach an undersea garden. The camera turns to the garden's owners, Mermaid Tasha and Mermaid Uniqua. A yellow fish is swimming towards the garden entrance, and the mermaids recognize the fish as a "garden intruder". They try telling it to go away, but fail. Uniqua tells Tasha to use a move used when trying to protect their garden, the "Mermaid Freak-Out".
They start blowing air through their lips, blowing a large current through the water. The fish spins three times and is blown away from the two. The area becomes dark as Pablo and Tyrone's submarine drives above the mermaids. The camera turns back to the explorers. On their sea life tracker, a red dot signals, alerting the boys that the possible mermaid is below them. Tyrone turns the submarine around, making it face the mermaids' garden. Tasha and Uniqua are terrified. At first sight, they thought the large yellow submarine was a fish. Now, they know what it is. A submarine. They look inside the glass window of the vessel.
They see the boys inside the sub and think that the pair wants to destroy their garden. The girls try hiding behind a rock to not be seen. They make mermaid calls. Tyrone and Pablo hear the noise coming from behind them and immediately turn their submarine around. Uniqua tells Tasha her plan to hide behind a rock in the opposite direction of their garden and make noises, causing the explorers to turn away from their home. Tasha agrees and does as Uniqua tells her to. Soon, Tyrone and Pablo are driving their source of underwater transportation away from the mermaids' coral garden.
Tasha and Uniqua sing Call of the Mermaid. Tyrone and Pablo start to sing as well. To make sure that Tyrone and Pablo don't come back, the mermaids start pushing the submarine away from their home. They struggle to hold the machine up. The camera turns to Tyrone and Pablo, who are confused about what is happening. Pablo asks Tyrone if he is causing them to rise up to the surface. Tyrone answers "no", and lets his hands off the wheel. Tasha and Uniqua push the two to a large area full of rock geysers that shoot water out of spouts. The girls place the sub on a geyser and swim away.
Tyrone and Pablo are worried that they will not be able to find a mermaid on their mission. The geyser spouts, shooting the submarine out of the water, interrupting the boys' conversation. The submarine quickly falls back in with a lot of force from the geyser's push. The submarine lands on the seabed, leaving the submarine unable to move. Tyrone and Pablo put on scuba gear and start their mission without their amazing submarine.
The camera turns to Tasha and Uniqua, proud of what they did to the explorers. They sing Our Garden to celebrate. Meanwhile, Pablo and Tyrone are swimming close to the garden. They spot the mermaids and Pablo tells Tyrone that he has to capture the mermaids on film. The mermaids misunderstand what Pablo said and think that he doesn't want to capture them on film, he wants to capture them for good.
The girls start to swim away, unaware of the undersea explorers gaining on them. Uniqua sees a small cave below and signals for Tasha to follow her. After the two swim into the dark rock cavern, Tyrone and Pablo do the same. All four find the cave very scary and dark. They sing You've Gotta Be Brave. Uniqua and Tasha come to a dead end and find nowhere to hide from the boys. Tyrone and Pablo swim close to them. The mermaids use the Mermaid Freak-Out on their enemies. The sound causes a rock avalanche from above. Many rocks start falling. The mermaids tell Tyrone and Pablo to get out of the rocks' way.
Pablo and Tyrone don't take mind in what the mermaids are saying. Pablo takes out his camera to get the talking mermaids on film. Tasha and Uniqua finally grab the two as the rocks collapse on the ocean floor. As they rise up to the way out of the cave created by the avalanche, the four emerge into the part of the sea above the cave - the mermaid garden. Tasha and Uniqua are furious and try telling the explorers to get out of their garden.
The girls now see the explorers as garden intruders. Tasha and Uniqua tell the boys that they are not to be captured and that their garden will stay unharmed. Pablo and Tyrone explain to them that they didn't want to destroy their garden or capture them. Pablo tells the two that he wanted to capture them on film with his camera, not capture them in cages. Tasha and Uniqua call the event a misunderstanding and allow Pablo to film them.
Pablo's stomach growls. Tasha thinks that the sound might be a whale. Uniqua thinks it is another intruder. Pablo tells that it was just his stomach. Tyrone tells the others that they can all have strawberries and yogurt. The ocean landscape transforms back into the backyard as the four sing the end song. They rush into Pablo's house for a snack. Uniqua and Tasha open the previously closed door and make the call of the mermaid: "Prrrt!"