May they all sink under my sorrow.
"Syren, I am Hawk Moth. I'm giving you the power to turn Paris into your own underwater kingdom for you and your prince. In return you must bring me Ladybug and Cat Noir's Miraculous."
— Hawk Moth
Ondine, a talented swimmer with a crush on Kim, is akumatized into Syren, a mermaid villain, who floods Paris to make it her underwater kingdom.
Tropes:
- Brand X: Marinette and Alya go to see a movie made by "Film Studio" and released by "Cinema Productions."
- The Cameo: Rena Rouge returns but doesn't play a large role in defeating the episode's main villain.
- Call-Back: Kim's crush on Chloé is brought up again.
- Cats Hate Water: After Ladybug quips that ladybugs can't swim, Cat Noir jokes that "cats aren't crazy about water either".
- Color-Coded for Your Convenience: To unlock the new powers of the kwamis, Marinette is given a bunch of potions to feed to Tikki. For the sake of portability, she decides to bake them into a bunch of macarons, each a different colour. Master Fu does the same for Adrien in the form of cheeses with coloured films.
- Continuity Nod: Markov appears by Max's side.
- Contrived Coincidence: Master Fu has nearly completed work on a recipe that will give Ladybug and Cat Noir aquatic powers, just in time for a villain that floods the city. Admittedly the special potions he's working are for a variety of powers, not just aquatic ones, but he specifically highlights the aquatic one when describing them to Marinette.
- Deadpan Snarker: Alix when she asks Alya if they are saved after seeing Marinette in the trash can.
- Dramatic Irony: The episode ends with Gabriel vowing to find the Guardian, unaware said Guardian is currently inside his house.
- Eye of Newt: Master Fu has an array of mundane and mildly fantastic ingredients for mixing the potions for kwami transformations.
- Food as Bribe: Adrien tries to make Plagg reveal the secrets Ladybug is hiding from him with promises of rare cheese. Though sorely tempted, Plagg refuses.
- Foreshadowing: Marinette hints that one of her new power-ups is "icy". We see this transformation in "Frozer".
- Friendship Moment: Plagg begs Adrien not to give up his ring, because even though Cat Noir is replaceable, Adrien isn't.
- Homefield Advantage: Ladybug and Cat Noir are unable to fight Syren underwater, which makes her impossible to catch as all of Paris is flooded. It takes a power up potion to even the odds. This is later inverted to take out Syren: she's fished out of her watery kingdom and onto a rooftop using a car, Ladybug's yoyo, and Cat Noir's staff.
- I Just Want to Be Special: Kim really wishes he could have super powers, to he point he finds the idea of Syren turning him into a merman tempting. His only problem is that he wouldn't be able to go to the movies.
- In Medias Res: The episode starts with Alya given another chance to be Rena Rouge, without any build-up. We have no idea how or why the animals from the zoo escaped.
- Internal Reveal: Adrien gets caught up on who Master Fu is and other details.
- Interrupted Declaration of Love: Ondine tries to tell Kim about her feelings for him, but she's interrupted by his watch alarm. She hints that this wasn't the first time.
- Keeping Secrets Sucks: Ladybug keeping Master Fu a secret from Cat Noir strains their relationship a little, but the latter is all caught up by the end of the episode.
- Literal Metaphor: The ingredients for the potion are all given overly elaborate, metaphor-laden, flowery descriptions. Master Fu assumes the "tear of joy" ingredient is no different, and tries everything he can think of before Marinette suggests it might be an actual tear of joy, which it is.
- Meaningful Name: "Ondine" is French for "Undine," a mythological term for water-based beings like mermaids and sea-nymphs. Sure enough, when she becomes an Akuma...
- New Powers as the Plot Demands: The villain floods Paris, putting Ladybug and Cat Noir at a disadvantage since neither of them can stay underwater for very long. Good thing Master Fu and Marinette were working on inducing kwami transformations, one of which is an Aqua mode! Said mode includes scalier redesigns, flippers, and webbed fingers. In addition, Ladybug's yo-yo can function as a rebreather.
- Oblivious to Love: Not played for laughs. Kim's inability to pick up on Ondine's hints that she likes him makes her so frustrated that she becomes vulnerable to Hawk Moth's corruption. The events of the episode cause Kim to catch on, and he asks her out.
- Ocular Gushers: The first thing Syren does after becoming akumatized is to turn these on. They flood all of Paris in a matter of minutes.
- Offscreen Teleportation: Just how the hell did Master Fu get on top of that building with Adrien without either him or Plagg noticing?
- O.O.C. Is Serious Business: Frustrated by Ladybug's secrets, Adrien threatens to throw away his ring if Plagg doesn't spill. Given how much he loves the freedom that being Cat Noir allows him, this shows a lot.
- Oxygenated Underwater Bubbles: Syren provides Kim with a wearable one, then puts him on a throne in a much larger one.
- Power Makes Your Hair Grow: One visual indicator that Marinette used the Aqua powerup is her longer pigtails, which are now pink at the tips.
- Power-Up Food: Kwami can be granted new powers and transformations by magical formulae. Master Fu gives Marinette and Adrien all the known mixtures, the former baking them into mini macaroons and the latter receiving them in the form of cheese from Fu.
- Properly Paranoid: Master Fu refuses to let the kwami see the transformation recipes for fear they would be forced to divulge that information if captured. Gabriel tries to do exactly that with Nooroo, who can't reveal what he doesn't know.
- Punny Name: Ondine. Undine are peaceful female water nymphs that can acquire souls by marrying human men, whereas a Siren is a hostile female creature associated with drowning men at sea. In French, the word for mermaid is sirène.
- Super Not-Drowning Skills: The main power of the Villain of the Week and given by the introduced Power-Up Food.
- Sunken City: What Paris is reduced to once Syren floods it, with survivors waiting on the rooftops for Ladybug and Cat Noir to right things.
- Speak Friend and Enter: The reason why it has taken so long to decipher the transformation recipes is because the Guardian was too caught up reading into the line "tears of joy". Instead of trying an actual tear of joy.
- Swiss-Army Tears: A tear of joy is the final ingredient for the transformation recipes. And in a separate example, Syren's tears flood all of Paris.
- Transformation Sequence: With Ladybug's new aquatic abilities comes a new transformation sequence. Cat Noir's, however, isn't seen.
- The Un-Reveal: Once again, Master Fu tries to transform only for it to fail, this time because he messed up his recipe.
- Water Is Womanly: As a civilian Ondine is a sweet, cheerful girl whose name is based on the feminine elemental of water Undine. When turned into Syren, she becomes a mermaid obsessed with making Kim her prince.
