The prequel film was released direct-to-DVD in 2008, and depicts events that happened to a young Ariel before she had above-the-water adventures and fell in love with a human prince. Most notably, the film shows us Ariel's mother, who's never been seen in any of the animated canon before, for the first time.
Initially, this film was supposed to be The Little Mermaid III and include Melody (check out this early teaser) but was reshaped into a full-on prequel. This film is the very last of Disney's direct-to-video sequels / prequels / in-between-quels phase that started in the 1990s with The Return of Jafar, and marks the end of an era. Actually, this film was still in production when the decision was made to close down that business, so Ariel's Beginning was completed in the knowledge that it would be the last of its kind.
There is already a prequel of the 1989 classic film in the form of the animated series, but this film is independent of that, and features details that are unique to itself, such as a different way that Ariel and Flounder met for the first time, different age order for Ariel's sisters, and a very different relationship between Ariel and Triton. Probably the best of these changes is its highlighting Ariel's relationships with her sisters, who truly feel like her family here.
Jodi Benson returned to reprise her role as Ariel, as does Samuel E. Wright as Sebastian, but the rest of the cast are new, including Jim Cummings as King Triton, Sally Field as Marina Del Ray, Parker Goris as Flounder, Tara Strong (who voiced Melody in Return to the Sea) as Adella and Andrina, Jennifer Hale as Alana, Grey DeLisle as Aquata and Arista, Kari Wahlgreen as Attina, Jeff Bennett as Benjamin, and Lorelei Hill Butters as Queen Athena.
The story of Ariel's Beginning focuses on the role of music in Atlantica. A prologue sequence shows us the merfolk enjoying music and lounging on the ocean surface. King Triton, Queen Athena and their young daughters are among them, but the joy is cut short when a pirate ship attacks, and Queen Athena is killed. Following this tragedy, King Triton in his heartbreak decrees that there shall be no more music nor merrymaking in Atlantica, and he emotionally distances himself from his daughters. (It's all very The Sound of Music.)
Ariel grows up in this stifling prohibition environment and, despite the advice of her sisters, yearns for more. It's tricky, because the seven daughters of Triton are being monitored by Marina Del Ray, technically their governess, who is a tad power hungry and wants to take over Sebastian's role as royal attache. But Ariel finds her way, meeting Flounder and discovering a secret music club where the Atlanticans may indulge in music to their hearts' content. This exciting secret seems enough for the moment, especially when Ariel can share it with her sisters (it's very The Twelve Dancing Princesses), but Marina is a suspicious one, and Triton has to find out the truth eventually. Ariel ends up being the key to opening Triton's heart again, and bringing joy back to Atlantica.
Music is a major theme of Ariel's Beginning, but sadly there's no official soundtrack album for the film.Song List
According to the media that came out after the original 1989 film (comics, books), the birth order of Triton's daughters is the order that they were introduced in the song "The Daughters of Triton" in the original film, i.e.:
Aquata, Andrina, Arista, Attina, Adella, Alana, Ariel.
While Ariel's Beginning changes the birth order of the seven sisters to this:
Attina, Alana, Adella, Aquata, Arista, Andrina, Ariel.
My guess is that for Ariel's Beginning they were ordered based on their headgear, so Attina, who has the large, dramatic spikes, becomes the eldest or crown princess.
Ariel's Beginning was released direct to DVD in August 2008. Extras on the DVD include deleted scenes, a "Disney Song Selection" sing-along feature, interactive games, and two Backstage Disney featurettes: one about the movie, and another about the Broadway musical. This same year, the film was bundled in a The Little Mermaid Trilogy set that has all three films.
In 2013 there came along two new releases for DVD and Blu-Ray, namely the 2-Movie Collection that bundles The Little Mermaid: Ariel's Beginning and The Little Mermaid II: Return to the Sea together, plus a The Little Mermaid Complete Collection that has all three films together.
Ariel's Beginning features some beautiful underwater animation that puts the series and some other direct-to-video films to shame, and it brings to the forefront Ariel's relationship with her sisters, which tends to be overlooked. But the film as a whole is a strange one.
Myself, like a lot of other fans, were excited about this film when it was announced, because there are so many things that can be explored about Ariel's youth and backstory. The film even does answer one major thing, by showing what happpened to Ariel's mother, but other than that, it's intent on answering questions people didn't ask (about the role of music in Atlantica, or how Sebastian became court composter), instead of questions that people do. There's no mention of Ariel's most famous trait, i.e. her fascination with the world above, or where that came from; there's no further exploration of Triton's decree to ban merfolk from the surface; Ursula makes no appearance whatsoever despite her saying in the 1989 film that she used to live in the palace.
On that last note about Ursula, some of us who anticipated this film thought that Marina Del Rey would be the key to Ursula's backstory. Check it out: Marina's power-hungry, lives in the palace, has a purple colour scheme, and has eels for sidekicks. I have no evidence for it whatsoever, but the sheer number of coincidences gets me feeling that Marina was supposed be connected to or transform outright into Ursula, but that was changed during production. In the end Marina turns out to be a minor nuisance with shallow goals, and doesn't contribute much to Ariel's journey.
The other strange thing about this film is that it ignores the prequel series. This isn't unusual (various Disney canons contradict each other all the time) but it does reveal a certain carelessness, because the series was groundbreaking as one of the first Disney television series to be based on classic films, and was the first time fans of Ariel got to see the little mermaid in action again canonically. The TV series is remembered for that, and was part of the franchise's fandom consciousness for sixteen years before Ariel's Beginning came along. Basically, we already had a depiction of Ariel's youth, which explored in far more detail Ariel's friendships and development of her human-world-interest hobbies, so it's odd that Ariel's Beginning would want to tread familiar ground yet not take full advantage of the gaps the TV series left behind, but instead rewrite over the series entirely.
Another major reason I'm not that keen on Ariel's Beginning's backstory, is its depiction of Ariel's relationship with her father. This film's version of as a hard-nosed fun-sucker makes me incredibly sad, for he loses ten years with his daughters because of his behaviour, and though I can believe their relationship is improved after the end of this movie, I'm far more attached to Ariel and Triton's relationship as seen in the TV series, where they have always been loving and close, and their disagreements stem from their equal stubborness, instead of Ariel being helpless in the face of Triton's emotional distance.
The interesting thing is, since the TV series hasn't been released on DVD in any meaningful way, I expect that in years to come, Ariel's Beginning may take over fans' memories as being more canonically "true", since the film is more accessible and prominent than the TV series. As for me, I've been attached to the TV series for too long to view Ariel's Beginning as more than a vague alternate universe story, which has its own charms but is ultimately to take a backseat to the TV series.