Charmed is a WB television series about a group of sisters who also happen to be witches.
It aired from 1998 to 2006, and since it was on mainstream telly, I've seen a couple of episodes
here and there so I know the gist of the story, which is basically there are three sisters
who run around saving the world
before bedtime with their witchly powers.
The sisters are Prue, Piper and Phoebe in the first half of the series; a cast change meant that the
line-up became Piper, Phoebe and Paige in the second half of the series. (It's a long story.)
Anyway, the opening episodes of Season 5 featured some mermaid bits. The two-parter's title is A Witch's Tail (most of their titles are puns) and in the first half, a mermaid named Miley shows up asking for help. In what is a nod to Hans Christian Andersen's "The Little Mermaid", Miley has made a deal with a Sea Hag to gain legs in order to find love. When the episode starts, the Hag claims that Miley has lost, and has to give up her immortality as part of the bargain. Miley escapes and asks the three witch sisters (aka "The Charmed Ones") to help her.
Part One ends with the middle sister of the Charmed Ones, Phoebe (Alyssa Milano), transformed into a mermaid herself. For reasons that would take too long to explain*, Phoebe doesn't want to be transformed back into a human, and swims off into the big blue ocean, leaving her sisters behind. Part Two of A Witch's Tail reunites Phoebe with her sisters as they try to convince her that life under the sea isn't better than anything they got up there.
Phoebe has a tragic on-again-off-again relationship with a half-demon named Cole, played by the very delicious Julian McMahon. The relationship was trouble from the very beginning because the Charmed Ones are the good guys and demons are the bad guys, but Cole is in a bit of a grey area because he's half-human and, when not under demon influence, a pretty decent guy. Phoebe and Cole end up getting married, but it all goes to hell (literally!) and by the end of Season 4, Phoebe is tired of all the heartache and wants a divorce pronto.
In A Witch's Tail, Cole shows up just before the judge signs the divorce papers, stopping the process because he wants to "work things out" with her. In response, Phoebe has a bit of a freakout. She wants to be rid of all the pain, and when her younger sister, Paige, casts a spell to track down Miley, Phoebe transforms into a mermaid because in her mind, all she wants to do is just swim away and not face all her problems. Phoebe swims in the ocean and is enchanted by the simplicity of the undersea world, so after Miley is rescued and the Sea Hag defeated, Phoebe refuses to return home.