It's Clean Up the Beach Day, and Dora and her best friend Boots are cleaning up garbage when they find a singing clam, who tells them the story of Mariana the Mermaid. Mariana lives in the Mermaid Kingdom at the bottom of the ocean and one day, a mean octopus dumped garbage on the kingdom, making all of the mermaids and mermen sick. When Mariana found a magic crown, she wished the garbage away and the kingdom was beautiful again. But after a huge storm washed the crown away, the octopus returned, dumping more garbage over their beautiful home. Dora and Boots want to save the Mermaid Kingdom and with the viewer's help, they find the crown washed ashore on the beach.
After checking the Map, the duo set off on a magical adventure to Mermaid Kingdom. They travel across the Sea Shell Bridge, join the pirate piggies in a conga line through Pirate Island, and call upon animal rescuer Diego to help them ride dolphins across the Silly Sea. Even Swiper helps them swipe garbage along the way.
When Dora and Boots finally make it to the Mermaid Kingdom, the octopus captures Mariana. So Dora puts on the crown and upon being granted her one wish to save the Mermaid Kingdom, she turns into a mermaid, herself.