It's Friday night and while Freddy and Daphne 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.
The episode provides examples of:
- Brick Joke: Early in the episode, Velma borrows a boat from Daphne’s family without asking, and it subsequently gets damaged by the fish people. Later, the whole gang travels to the platform with the same boat and Daphne comments that “Daddy’s little boat” seems more dinged up than she remembers.
- Chekhov's Skill: Shaggy and Scooby's ability to hold breath to eat lots of food.
- Fanservice: While the mermaid looks very vaguely humanoid, we do get a glimpse of the truly beautiful woman inside wearing a bikini.
- Freeze-Frame Bonus: Velma's blog contains a comment from Shaggy recommending a pizza place, followed by a comment from The Creeper complaining about how much the pizza in prison sucks. Directly above it is Fred asking her to pick up "some extra 30 weight, 6 car batteries with jumper cables and a trombone" for a new trap.
- 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.
- Homoerotic Subtext: According to the creators, the fact that Velma finds herself drawn to an attractive woman and forms a clear bond with her right after her breakup with Shaggy was intentional.
- 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."
- Monster of the Week: The Fish Freaks, gillman-like creatures that haunt the waters around Destroido's abandoned drilling platform.
- "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.
- Spot the Thread: Velma realises the truth about Angel when told that the disasters she shared information on were covered up by Destroido.
- Take That, Scrappy!: Invoked by Daphne and Fred against the actual Scrappy. "Look away, Daphne! We promised we'd never speak of it again."note
- Also against Flim-Flam. "Twenty-five to life is a long time, even for a pre-teen con artist."
- Unwilling Suspension: The gang is subjected to this by the Fish Freaks after being captured.
- Wham Episode: The end cements this as one, as we get a very big twist: Angel Dynamite is actually Cassidy Williams, one of the original Mystery Inc.
- Velma: Things have been adding up for me, Angel. The help you gave us on this oil platform mystery, the diary you provided for the Mystery Manor case; stuff that you shouldn't know and shouldn't have had, no matter what the heebedy-jeebedy.
Angel: What are you saying?
Velma: I'm saying Angel Dynamite isn't your real name. I think your real name is Cassidy Williams, and you're one of the members of the original Mystery Incorporated that vanished in the caves beneath Crystal Cove. Tell me I'm wrong...friend.
