During a vacation to Merroway Cove on the family's "Floating Palace", Sofia befriends a mermaid and works with her to stop her family's longtime war against the human race.
Tropes:
- Big Bad: Cedric in a sea monster form threatens harm to both Oona and Sofia in an attempt to get the former's magic comb (albeit Sofia doesn't recognize him). After he is defeated, Queen Emmaline takes his place, when her extreme prejudice towards humans leads her to attempt to sink the Floating Palace.
- Big Damn Heroes: Cora appearing and using the power of Oona's comb to defeat Sea Monster Cedric, saving Oona and Sofia just in time.
- Cassandra Truth: When Sofia tells her family what happened to Oona, they don't believe her and they brush it off as the mermaid story she read in her book. She asks Clover and Farley for help instead, who readily believe her because Clover was there when Sofia first met Oona.
- Damsel in Distress: Oona becomes this when Cedric holds her hostage for her comb.
- Disc-One Final Boss: Sea Monster Cedric is defeated before the final third of the special, just before facing Queen Emmaline and stopping her from destroying the Floating Palace.
- Extra-Long Episode: The first of the series proper, following the Pilot Movie Once Upon a Princess. Disney+ splits it into a two-parter episode, however.
- Failed a Spot Check: Sofia never recognizes the sea monster as Cedric, despite Cedric retaining his hairstyle and not changing his voice.
- Fantastic Racism: Plank harbors a strong grudge against humans for always disrupting their kingdom, similar to King Triton's grudge.
- Fighting Your Friend: For the first time, Sofia has to fight Cedric; however, because Cedric has transformed himself into a sea monster, Sofia doesn't recognize him.
- Insistent Terminology: Oona prefers calling the shipwreck of the humans who captured the mermaids her "hideaway".
- Knight of Cerebus: Queen Emmaline is the first antagonist ever to be even more serious than Cedric, given her vengeance against humans for always casting her kingdom in shadow that she threatens to destroy the Floating Palace and drown the Royal Family still on it.
- MacGuffin: Oona's comb. Cedric plans to steal it to take over Enchancia with it since he failed to steal the Amulet many times.
- Making a Splash: The comb Oona wears carries powerful underwater magic, but she cannot unleash it until she's old enough.
- Mass "Oh, Crap!": The royal family all have this reaction when they see Queen Emmaline's storm rolling in.
- Mermanity Ensues: Sofia gains this power as a reward for rescuing Oona from a net; she turns into a mermaid once she puts her feet in the water.
- Not Quite Back to Normal: When Cedric and Wormwood turn back to normal, Cedric still has his tail.
- Race Against the Clock: Queen Emmaline only gives Sofia until sundown to rescue Oona before the queen creates a storm with her trident to sink the floating palace and get Oona back, believing that that's where she's being held captive.
- Secret-Keeper: Oona becomes the first to hear of the Amulet of Avalor and its powers.
- Water Is Dry: Averted whenever Sofia reverts to human form after becoming a mermaid; she stays wet as a human, which is noted by Roland.
