Scooby Doo Mystery Inc/Recap/S1 E20 The Sirens Song

It's Friday night and while Freddy and Velma are on a date and Shaggy and Scooby are pigging out, Velma is bored and wants a mystery. When she gets an anonymous tip about an abandoned oil rig and a missing sardine boat, she goes to investigate alone...and finds the mysterious tipster is actually a mermaid.

Tropes used in "The Siren's Song"

 * Enthralling Siren: Though she's referred to as a mermaid, she has an enthralling voice so she's meant to evoke this.
 * Green-Eyed Monster: Part of the plot is pushed by Velma feeling like a fifth wheel - Daphne has Freddy and Shaggy has Scooby, while she has no one to hang out with on Fridays.
 * Logical Fallacies: "Sometimes you have to kill the environment to save it!" Mystery Inc all together: "Uh, no, I don't think that's right."
 * Scooby-Doo Hoax: Of course - except this episode has both the villains and the good guy (Amy the mermaid) perpetrating a hoax for two different reasons.
 * Sirens Are Mermaids: Amy is always referred to as a "mermaid" and never as a "siren," yet the episode's called "The Siren's Song." Perhaps because "The Mermaid's Song" doesn't have the same ring to it.
 * Take That Scrappy: Literally. "Look away, Daphne! We promised we'd never speak of it again."
 * Also against Flim-Flam. "Twenty-five to life is a long time, even for a pre-teen con artist."
 * Wham! Episode: The end cements this as one, as we get a very big twist: