The post-apocalyptic sci-fi television show Dark Angel follows the adventures of Max Guevara (Jessica Alba), a genetically-engineered soldier who escaped from and is trying to stay hidden from the people that created her for the Manticore program.
One of things things Max does is aid and rescue fellow Manticore escapees like herself. In the season 2 episode, "Gill Girl", she discovers a Manticore "mermaid" (Anita Brown) who is being kept inside the tank of a strip bar. The mermaid, whose name we don't get, was snagged in a fishermen's net and sold to the bar as an attraction. She may have legs like regular people, but she also has webbed hands, can only speak in shrill dolphin-like tones, and has gills just along the side/under her ribs that mean that she can only breathe underwater. Max makes it her mission to rescue the mermaid, and along the way teams up with the mermaid's mate, a merman (Jesse Hutch), who has risked travelling on land to rescue her.
The episode also calls back on Hans Christian Andersen's "The Little Mermaid" starting with the opening scene where Max reads the book to her friend/love interest Logan's niece. Max wraps up the story with a happy ending where the mermaid got to see her family again, but after the child leaves Logan points out that the story ends quite differently. Max protests that she prefers the happy ending — after all, this is a pretty dismal future they're in, and sometimes just believing in a happy ending is all they have to cling to. Max repeats this later on in the episode while she and her other friend Alec are helping the mer-pair escape from the bad guys, where Max declares stubbornly that she is going to fight for the happy ending, no matter what it takes or how harsh the odds. It's a pretty neat thematic use of the fairytale and the power that stories have over us.
Thanks to Martha for letting me know about this episode!