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|“|| Huh! This is amazing! I can breathe underwater, just like a fish.
I knew you'd love it, and I've got lots of other surprises for you, you'll see--
Nadja Chamack reports on several escaped zoo animals rampaging across Paris, while Ladybug, Cat Noir, and their new friend Rena Rouge try to recapture them. Meanwhile, Ondine and Kim are at the swimming pool. Kim wonders what it's like to be a superhero, such as the powers, the secrets and the costumes a superhero would be wearing. Ondine takes out a note and puts it in the capsule of her bracelet. She then turns to Kim and asks if he wants to know her secret, before throwing the bracelet in the water.
Kim dives in, retrieves the bracelet, and reads the note inside, which says that Ondine has a huge crush on a boy who has no idea about it. Oblivious, Kim says that the boy is clueless. He then puts his own note in her bracelet and throws it in the water for Ondine. She reads the note that says Kim can stick his big toe in his ear. Ondine decides to state outright who she's crushing on in her next note. Just as Kim is about to open the note, his watch alerts him that he's going to be late for the movies. Kim tells Ondine that he asked Chloé to go to the movie with him, and dashes off. Ondine is left on the poolside, lamenting on how Kim was the one she had a crush on.
Hawk Moth's lair opens up as Hawk Moth senses Ondine's sadness. He remarks on how many times Ondine has tried to tell "her prince" how she feels, only to be dismissed, before he creates an akuma and sends it to akumatize her.
Meanwhile, Cat Noir is being chased by a pair of black panthers, and he jokingly asks them if they would really eat one of their cousins. Rena Rouge then uses her Mirage power to create a series of steak illusions. The panthers fall for the mirages, and are drawn into and trapped in the back of a van. Cat Noir prepares to fist-bump Ladybug and Rena Rouge, but they have to leave because Rena Rouge is about to transform back. Upset, Cat Noir asks why Ladybug is allowed to know who Rena Rouge really is when he isn't. Ladybug explains that she must return her Miraculous. Cat Noir asks if his role is to be left in the dark about Ladybug's secrets, and Ladybug says she will talk to "him" about it. Cat Noir asks who "him" is, but Ladybug says that must wait until the time is right, and leaves with Rena Rouge. Frustrated, Cat Noir declares that he hates secrets.
Rena Rouge soon detransforms back into Alya, and hands her Miraculous back to Ladybug. Alya offers to team up with Ladybug again, before her phone alerts her that she's late to the movies, and she runs off. Ladybug turns back into Marinette and begins running in the opposite direction from Alya. Tikki advises her to take Alya's route in order to get to the movie, but Marinette opts to miss the trailers again. Overhead, the akuma flies into the swimming pool and attaches itself to Ondine's bracelet. Hawk Moth offers Ondine the power to turn Paris into an underwater kingdom for her and her prince in exchange for the Ladybug and Cat Miraculouses. Ondine accepts the deal and turns into Syren, whose tears then fall into the pool, making it start to overflow.
Kim runs up to the cinema, where Max and Markov are waiting for him. Kim asks if Chloé came, but Max jokingly asks if he really thought she would come see Mecha Monkey vs. Cyber Sharks 3. Alya runs up to the two boys and asks if Marinette is late too, which Max confirms. Alya stands outside to wait for Marinette, while Max, Kim and Markov enter the theater, Max promising to save seats for the girls. Meanwhile, water floods out of the swimming pool building.
Marinette returns the Fox Miraculous to Master Fu at his massage shop, and Master Fu asks what's troubling her. As they enter the next room, Fu prohibits Tikki and Wayzz from following, as kwamis aren't allowed to know the recipes that can modify their powers, in case they are captured like Nooroo. In the next room, Marinette explains that Cat Noir is asking more questions and is tired of being left in the dark, but Fu insists that they must wait for the right time to tell him the truth, comparing it to putting noodles in boiling water. He then shows that he is unlocking the recipes to give her and Cat Noir new powers; however, he is unable to figure out what ingredient the "Tear of Joy" is. Thinking the Tear could be honeydew, Fu adds it to his brew, and believes he has unlocked underwater powers. As this happens, people beside the Seine run away as it begins to overflow.
At the Agreste mansion, Adrien is in his room, playing with Cat Noir and Ladybug action figures. Adrien expresses his frustration with Ladybug not trusting him through the figures. Adrien turns to Plagg and asks if he knows what Ladybug is hiding from him, but Plagg refuses to tell. Adrien then begins offering to order Plagg different types of cheese if he tells the truth.
Back at the massage shop, Fu feeds the potion to Wayzz, who believes it to be the right one. Excited, Fu tries to invoke a transformation in Wayzz, but nothing happens, and Wayzz suggests adding a little more salt. Marinette gets a call from Alya, and rushes off to the cinema, leaving Fu and Wayzz.
Marinette arrives at the cinema and enters with Alya, unaware of the water running through the streets behind them. Rose, Nino and Juleka wave Alya and Marinette down, and they shuffle into their seats. As the movie starts, Marinette notices people running away in fear, and realizes that the cinema is flooding with water. Outside the cinema, water rushes through the streets and floods them. Back in Adrien's room, Adrien is still trying to bribe Plagg when he notices that the room is filling with water. He transforms into Cat Noir.
Marinette runs up to the roof of the building where the others are looking to see that all of Paris has been flooded. Markov claims that his circuts are sensitive to water. Unconcerned, Kim jumps in the water and says he can teach Markov how to swim. A fin swims towards Kim, and Syren jumps from the water, and takes him under. While her friends try to look for Kim, Marinette gets into the water in a recycling bin and paddles away using a shovel, claiming she is going to get help. Cynically, Alix asks if this means they're saved.
Underwater, Syren grasps Kim and creates an air bubble for him to breathe in. Marinette transforms and searches for Kim underwater. Ladybug soon spots him with Syren, who is showing Kim around the submerged Paris, much to his amazement. Ladybug latches onto Kim with her yo-yo, but Syren tries to pull him back. Hawk Moth boasts that every new environment brings a new set of rules, and Ladybug can't win because she can't breathe underwater. Cat Noir is then seen leaping across the rooftops above the water, when he sees Ladybug flying through the air, having been knocked out of the water by Syren. Cat Noir saves Ladybug, and asks if there's a chance she can get them a submarine. Ladybug uses her Lucky Charm, and receives a pot. Realizing the pot indicates Master Fu, Ladybug says she has to leave. Cat Noir is frustrated at Ladybug not telling him where she's going, despite them being a team, but Ladybug insists it isn't easy for her, either. She then zips away, while Cat Noir is left behind, further frustrated.
Ladybug detransforms and finds Master Fu on the roof of his building. Marinette tells Fu that she and Cat Noir can't beat Syren underwater, which he knows, although he can't find the right blend that could give them underwater powers, believing he has failed them. Marinette suggests using a real tear of joy in the recipe, and tells Master Fu a series of jokes, making him shed a tear in laughter. Marinette takes the tear and drops it into the pot, which releases a burst of green energy. Excited, Fu says Marinette has found the final ingredient. Fu immediately feeds the potion to Tikki, who, on Marinette's command, powers up into Aqua Tikki. Marinette then transforms into a new version of Ladybug with aquatic powers. Before leaving, Ladybug turns to Fu and tells him its time to "put the noodles in the boiling water", which he agrees to.
Meanwhile, fed up with waiting, Cat Noir transforms back into Adrien and threatens to quit being Cat Noir if Plagg doesn't tell him what Ladybug has been hiding. Plagg says that Adrien mustn't do that, but Adrien insists no one will know or care if he does. Plagg says he would care, although Adrien dismisses this as being because there will be no one to give him Camembert. Plagg states that he's sure there will be another Cat Noir to feed him cheese, but they won't be Adrien. Adrien cheers up because of this, just before Master Fu, addressing him as Cat Noir, appears beside him. Adrien recalls how he had helped Master Fu on the first day of school, and Fu hands him a vial of the aqua-power potion, telling him he must help Ladybug. Plagg moans about how he hates transforming.
Inside the submerged swimming pool facility, Kim, inside a giant bubble, remarks that he loves what Syren has done to the place. She says that the pool is where she had tried to pour her heart out to Kim many times, and, after seeing her bracelet, he realizes Syren is actually Ondine. Syren says that she can turn Kim into a merman with a kiss, so they can rule their underwater kingdom together forever. Before this happens, Kim asks if that means he'll never get to see Mecha Monkey vs. Cyber Sharks 3, confusing Syren. Ladybug swims in and tells Kim he will get to see the movie, and Cat Noir joins her. Hawk Moth confirms his suspicions of the Miraculouses possessing hidden powers, and tells Syren to take them before Ladybug and Cat Noir steal Kim away from her. The heroes begin to fight Syren using their new abilities. Kim tells Ladybug how cool their new outfits are, and she asks what got his friend so worked up. Kim explains that he and Ondine were playing a game of Secrets in which she tried to tell him that he was the boy she liked, but he didn't realize it at the time. Realizing the bracelet must be where the akuma is hidden, Ladybug uses her Aqua Lucky Charm and summons a car key.
Ladybug gives Kim her yo-yo to breathe through, and he escapes with her while Syren and Cat Noir are fighting. Syren pursues them and demands that Ladybug tell her where Kim is, unaware he is on a nearby roof above them. Syren chases after Ladybug through several floating cars, only to be trapped inside one by Cat Noir when he strikes the car with his staff, setting off its airbag, and pinning her inside. Ladybug locks Syren inside the car with the key, and Cat Noir fishes the car out of the water using her yo-yo string and his staff. Ladybug unlocks the car and breaks the bracelet, releasing the akuma, which she then purifies. She throws the key in the air, returning the city to normal and de-akumatizing Syren into Ondine. Cat Noir thanks Ladybug for keeping her promise, and the two of them fist-bump.
At the massage shop, Fu gives Marinette seven augmented-power blends in different vials, which he tells her to keep with her at all times. Marinette thinks this is not very practical, and Fu says that only the ingredients matter, and that she can put them in anything, which gives her an idea. Marinette bakes some macarons at her house, but when Tom tries to eat one, she says she'll taste them first. In Marinette's room, Tikki says she loves the idea of the magical macarons as Marinette places them in boxes. Meanwhile, at the pool, Kim hands Ondine a bracelet, asking if she wants to play a game of Secrets. Ondine opens the bracelet and reads the note, which asks her if she wants to go to the movies.
At the Agreste mansion, Gabriel and Nathalie are inside Gabriel's atelier when they hear the doorbell. Gabriel uses a camera to see who is there, and finds Master Wang Fu. Fu, using the name Mr. Chan, claims to be a substitute for Adrien's Chinese tutor. Adrien is practicing his Chinese when Nathalie brings Fu into his room, much to his confusion. Plagg introduces Adrien to Master Fu as the last Guardian of the Miraculous.
Gabriel goes into his lair and interrogates Nooroo on how he can get new powers. Nooroo says that kwamis don't know the recipes. Gabriel deduces that the recipes must be inside the Miraculous spellbook and asks if Nooroo can read it. Nooroo says that only a Guardian can read the recipes and that he knows nothing more. Gabriel declares that he will find the Guardian, wherever he is. As this is happening, Fu hands Adrien a box of cheeses imbued with the different potions. Plagg moans that he hates transforming before the end card appears.
- Marinette Dupain-Cheng/Ladybug/Aqua Ladybug
- Adrien Agreste/Cat Noir/Aqua Cat Noir
- Gabriel Agreste/Hawk Moth
- Tikki/Aqua Tikki
- Plagg/Aqua Plagg
- Lê Chiến Kim
- Wang Fu
- Nadja Chamack
- Alya Césaire/Rena Rouge
- Chloé Bourgeois (mentioned)
- Trixx (background)
- Max Kanté
- Piáo Chóng (pictured)
- Rose Lavillant
- Juleka Couffaine (background)
- Nino Lahiffe
- Alix Kubdel
- Nathalie Sancoeur
- Adrien's bodyguard (flashback)
- Tom Dupain
- Emilie Agreste (pictured)
- This episode was first revealed in a TV guide from the UK.
- The episode title, Syren, is derived from the word "Siren."
- This episode was originally supposed to air on April 21, 2018, in the UK, but didn't due to a sudden change of schedule. It was later rescheduled for May 12, 2018.
- This episode marks the second time Adrien meets Master Fu. Adrien learns of Fu's identity as the Guardian of the Miraculouses and the one who gave Adrien's Miraculous to him.
- When Master Fu first addresses Adrien as Cat Noir, a flashback scene from "Ladybug and Cat Noir" is shown.
- This episode shows us that Rena Rouge does return to assist Ladybug and Cat Noir during difficult missions; however, she still has to give back the Fox Miraculous afterward.
- This episode marks the debut of the underwater power-ups, with indications of six other types. Marinette hints at the blue one having "icy" properties, a foreshadowing to "Frozer".
- This is the first episode that features a Lucky Charm item without the ladybug pattern.
- This is the third episode in which Cat Noir doesn't invoke his Cataclysm, following "Darkblade" and "The Mime".
- This episode shows the third highest number of kwamis seen in a single episode, with five kwamis: Tikki, Plagg, Trixx, Nooroo and Wayzz following behind "Heroes' Day" and "Sandboy".
- Trixx, however, has no speaking lines in this episode.
- This episode features the first time Adrien considers quitting being Cat Noir following "Stoneheart" when Marinette wanted to quit being Ladybug.
- This episode also has Syren as the second villain who refers to their love for their prince, which was also seen in "Princess Fragrance".
- The scene where Hawk Moth talks to Syren while explaining his powers given to her is reused from the episode "Pixelator", however with just a minor addition of the Ladybug and Cat Miraculous icons implemented. The mismatch between the audio and the visual animation is noticeable when watching Hawk Moth's lip movements.
- In the end card of the episode, Ladybug's face in the French dub is different than that when the episode premiered in Spain.
- In the French dub, the transformation music for Ladybug's in aqua transformation doesn't change.
- This episode reveals that the zoo has another panther since the one in "Animan" and "Puppeteer".
- Ondine, like Kim, was akumatized because the person she liked didn't reciprocate her feelings for them at first.
- When Hawk Moth says "Poor little mermaid", it is a reference to the Disney movie, "The Little Mermaid".
- Just before Rena Rouge transforms back, her skin tone is lighter than it's supposed to be.
- Rena Rouge has black accents at the bottom of her mask, which doesn't match her appearance in "Sapotis."
- When Kim first arrives at the movies, he's wearing his swimming cap without the goggles, but the goggles appear on the cap in the shot where he pulls the cap off.
- Outside the movie theater, the sign on the front that says Le Sorbonne is absent when Max points at the movie poster.
- The Bee Miraculous is shown differently from how it's supposed to be as shown in "Sapotis".
- Because the scene where Tikki enters the Ladybug Miraculous is reused from the regular transformation sequence, she is seen in her normal form instead of her aquatic form.
- When Ladybug has her aquatic outfit, a few of the spots on her suit are missing in some shots.
- When Cat Noir first arrives in his aquatic form, and when he positions himself right next to Ladybug, the scales and spots on Ladybug's mask are missing.
- When Ladybug opens her yo-yo to capture the akuma, she is wearing her normal outfit for that shot instead of her aquatic outfit.
- Similarly, in the shot where Ladybug and Cat Noir fist-bump, Cat Noir is in his regular outfit instead of his aquatic one.
- Gabriel's hair is miscolored grey.
- When Marinette finishes making the mini macaroons, her earrings are miscolored white.
- When Gabriel is talking to Nooroo in Hawk Moth's lair, the room is devoid of butterflies.
- Syren's eyelashes are missing when she shows her bracelet to Kim, after which they re-appear.
- During the Aqua Lucky Charm, when the ladybugs spiral out of Ladybug's yo-yo, they aren't lined up correctly with the yo-yo.
- In one shot Syren is shown to have feet.
- When Ladybug de-evilizes the akuma, her earrings disappear and the background is a bit darker than usual, as well as the yo-yo string in some shots.
- Ladybug's earrings also disappear in her aqua outfit for the rest of the transformation sequence.
- Plagg is shown to like the stench of Adrien's sock despite hating the smell of his shoe in the Season 1 episode, "Horrificator."
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