The story follows the sildroher, Ulla, a talented singer. For the sildrohers, music is a form of magic that can be used to summon storms, create fantastic objects, and even convert themselves to humans. Ulla has spent her childhood feeling like an outcast, having heard rumors of her being part human. This changes one day she is assigned in class to be the music partner of Signy, another talented singer with bright red hair. The two girls are able to summon powerful storms together and soon become close friends. Years later at a celebration of the king's youngest son, Roffe, the girl's gain the prince's attention by creating a magnificent garden for the royal family through one of their songs, composed by Ulla. Roffe is impressed by the power the two girls displayed and the three form a friendship, in which Ulla discovers that while the prince is lazy, he is dangerously ambitious. Signy falls in love with the prince, despite Ulla's reminders that he is a prince while they are commoners.
Every summer the sildroher princes take a three month long trip to the mainland where they mingle among the human nobility. The year the sildroher king declares his abdication of the throne, Roffe invites the Ulla and Signy to join him and his brothers on their trip to the human world. When they return the brothers will present gifts to their father, and the prince with the most impressive gift, regardless of birth order, will become the next king. While Signy hopes that Roffe may be developing feelings for her, Ulla knows that Roffe intends for the two girls to create a gift impressive enough to make him king.
Ulla's parents disapprove of their daughter going to the land, but Ulla joins Signy and Roffe regardless. The process of turning into a human involves violently cutting their tail in half while singing a spell that converts their bodies into human forms. The knives used in the transformation then have to be preserved and untouched by the mortal or else the sildroher would be unable to transfigure back to their original forms. After this is done the sildroher make their way to their lodgings with the royal family. The company spends the next three months mingling amongst the humans, attending dances, having affairs, and enjoying the vices of the mortal world.
While on land Ulla meets the apprentice to the king's seer who shows her a magical mirror in which a person's reflection can talk back to the viewer. As the trip progresses, Roffe becomes more eager for the two girls to create a gift that would secure his place on the throne. In an argument Ulla says that the only thing Roffe can give his father to win the title is fire that can burn underwater. Ulla believes this is impossible, but Roffe is adamant that it needs to happen. Ulla begrudgingly returns to the apprentice to ask him how to create such a fire. The apprentice replies with a grim response, the only way to sustain the fire is to take the breath, or life, of a human. The apprentice also reveals to Ulla that they are brother and sister, their mother being a Grisha who was impregnated by Ulla's father, a sildroher. The apprentice invites Ulla to leave with him to return to their mother, saying that the sildroher will never show Ulla the love and devotion she craves. Both sets of information devastates Ulla.
Now knowing the rumors surrounding her birth are true, Ulla asks Signy if she would still love her even if she were part witch, to which Signy assures she would always love her. Ulla is determined not to create the fire, but Roffe bargains that if Ulla creates the fire he will marry Signy and make her his queen. As the weeks go by Roffe pretends to be falling in love with Signy, causing her hope to rise. Unable to break her best friend's heart, Ulla agrees to go through with the murderous ritual. On the last night of their trip, Roffe drugs a ten year old he claims to be a murderer and the two girls begin singing their spell. The boy awakens right as Roffe cuts out the child's lungs, then setting them on fire to create the eternal flame. In the process Ulla catches fire and is left in critical conditions.
When the ritual is complete, Roffe reveals that the sacrificed boy wasn't a murderer at all, but an innocent child. He also reveals that he used Ulla's knife to cut out the boy's lungs, meaning that Ulla would be unable to transform back into a sildroher. Ulla realizes that the only person who had access to the trunk Ulla kept her knife in was Signy, who admitted to giving the knife to Roffe who told her he would only use it as leverage in case Ulla wanted to back out of the ritual. Mortally injured and bleeding out on the floor, Ulla begs Signy to summon the other sildrohen to heal her, but Signy instead chooses to leave with Roffe, abandoning Ulla to die.
Angry at Signy's betrayal, Ulla wills herself to find the apprentice's mirror. She holds a second mirror up to her reflection creating an unending chain of reflections and has all of them sing a healing spell to restore her. Ulla then changed the song to summon a storm that brought the ocean up to the palace, destroying the town, much of northern Fjerda, the sildroher's kingdom, and returning her to the sea. After the storms passed, Roffe and Signy became king and queen, relocated their kingdom, and had seven daughters, but the sildrohers stopped visiting the shore and using their singing magic ultimately forgetting their history. Ulla lives in the caves and pools of the northern islands awaiting Signy's daughters to come to her, like so many other ambitious people, to strike bargains for the things they desire in return for the things they hold dear.
- Ulla: a sildroher with a talent for singing and composing musical spells.
- Signy: a sildhroher with fiery red hair who shares Ulla's talent for singing.
- Roffe: a sildhroher prince with golden hair.
- The seer's apprentice: a mysterious man who resembles Ulla. He is later revealed to be her half-brother, as their mother was a Grisha who was impregnated by a sildroher.
- Ulla's father: a sildroher who seeked out on land in search of a wife. He was there when Ulla and the seer's apprentice were having a conversation.
- When Water Sang Fire is a retelling of The Little Mermaid told in the form of an origin story.
- In the author's note of The Language of Thorns, Bardugo writes that while the Hans Christian Andersen story was the point of departure for her retelling, "it's worth mentioning that Ulla is the Swedish diminutive of Ursula" implying that the sea witch from the Disney adaptation served as inspiration as well.
- The scholar is described as having shadows shifting around him, pulled like a tide. Given Bardugo's tendency to cameo her characters, this may or may not be the Darkling. She has also mentioned that "one of the characters you've met from the Grishaverse does make a brief appearance" in the Language of Thorns collection.