Sofia the First is an on-going animated musical TV series about Princess Sofia, a regular girl who becomes a princess when her mother marries the king. A pilot special first aired in November 2012, and the series proper started in January 2013. In November 2013, during the hiatus between Season 1 and 2, another special was released: Sofia the First: The Floating Palace.
In this special, Sofia and her family have taken their floating palace to Merroway Cove for a holiday. There are stories that merfolk live at Merroway Cove, though practically everyone around Sofia says that they're just a myth. As these things happen, Sofia meets a mermaid name Oona and they become friends. Sofia even gets a tail of her own, as a magical reward from her amulet for helping Oona.
Sofia would love to see where they live, so the pair travel underwater together to a kingdom that's not quite like Atlantica (a not-too-distant cousin, maybe), where the merfolk are suspicious of the humans who have arrived at the Cove. Oona's mother, Queen Emmaline, is wary despite Sofia's explanation that they mean no harm. Things get worse for Sofia when Oona gets kidnapped, Sofia's family refuse to believe her about the merfolk, and Queen Emmaline threatens to conjure up a storm and sink the floating palace if Oona isn't returned to her safely.
It's up to Sofia to figure out how to save the day, with a little helpful advice from Ariel, who gets summoned to the scene by Sofia's magical amulet. Ariel and Sofia even get to sing together in the song "The Love We Share".
As a meta bonus pointed out by Elphie in the comments, Queen Emmaline is also voiced by Jodi Benson, which is a really nice touch. I was also tickled and delighted by Sven the seahorse's alarmed, "Princess Ariel?" upon seeing her, because of course Ariel is a celebrity mermaid in the seven seas, yo. Queen Emmaline's awesome trident got pleased applause from me, too.