Plot
Geralt is hired to kill an Allamorax which is deemed responsible for the deaths of several fishermen in Bremervoord. He lets the creature go after the sirens beg to have mercy on it and claim that the Allamorax is not responsible. Since Geralt forfeited the kill and the prize money, Jaskier accepts an invite to play at the local festival to earn money. There, they meet Jaskier's childhood friend and bard Essi 'Little Eye' Daven. King Usveldt and his son prince Agloval attend as well, along with the king's bastard son Zelest, who used to bully Jaskier when they were young. During the festival, a boat with pearl hunters is attacked. One of the fishermen makes it to the festival and before dying, says they were attacked by Vodnik (humanoid fish creatures) on the sirens' order. Usveldt has originally refused to take action against the sirens because his son is in love with the siren princess, Sh'eenaz. However, this attack finally begets retaliation, with Usveldt ordering Geralt to find and hunt the ones responsible for killing the pearl divers. Also during the festivites, Usveldt introduces a young human princess to Agloval, hoping he will forget about Sh'eenaz. Meanwhile, the sirens are calling to their king Basim and queen Dahut for action, led by Melusina; Dahut's sister and Sh'eenaz's aunt, as the humans are taking away their food due to the excess fishing and profit of oyster pearls. Sh'eenaz manages to quell the restlessness of her people, claiming a marriage with Agloval could bring peace to both worlds.
Geralt, Jaskier and Essi investigate the attack further and find the Vodnik's lair. They confront a Vodnik assassin sent to hunt Geralt as to who hired them to attack the pearl hunters, but before they can get any information out of them, Zelest and his army arrive and kill the Vodnik, igniting a battle between Zelest's troops and the other Vodnik in the area. Agloval and Sh'eenaz watch the battle from afar in their secret meeting spot, with Geralt, Jaskier and Essi catching up with them after Zelest retreats. Agloval wants to marry Sh'eenaz, but through an argument translated by Essi who understands the mermaid language, they argue and part ways in anger, exacerbating the war between both parties. Because of a comment Jaskier makes; and after investigating the shipwrecked pearl hunter's boat with Essi, Geralt realizes the true villain is Melusina, who is revealed as a sea witch. Using shape-shifting magic, Melusina attacked the boats by transforming into a Kraken while also impersonating as the human princess Usveldt wants Agloval to marry. Furthermore, Usveldt has been collaborating with Melusina in order to convince his son that Sh'eenaz is not a worthy bride, with Melusina wanting to get back at her sister after King Basim chose Dahut as his queen instead of her. Melusina flees the palace after she is exposed, but Usveldt still intends to go on with the war and has Geralt and Jaskier arrested along with Zelest, who was unaware of the conspiracy and goes against his father. As the mermaids prepare for the upcoming battle, Melusina secretly offers Sh'eenaz a potion to transform her into her lover's form, giving her the chance to be with Agloval as a human. Essi manages to evade capture and privately speak to Agloval, downtrodden due to his father's deception, giving him words of encouragement and persuading him to defy his father and end the war. Usveldt sails into the mermaid's waters to attack them, with Geralt and company in tow as hostages.
Geralt, Jaskier and Zelest manage to escape and join the fight, with Melusina once again transforming into a kraken and attacking everyone, killing Zelest in the process. Usveldt wounds Sh'eenaz and she retreats into the sea, as Geralt is swallowed by Melusina. Geralt escapes by cutting her open from the inside, killing Melusina with a fatal blow to the kraken's head. Arriving with Essi on a small ship, Agloval manages to stop the fighting and exposes the deceit from his father and what his actions have caused, while Sh'eenaz is rescued and healed by Dahut. Sh'eenaz and Agloval agree to marry and bring peace to both kingdoms.
On the wedding day, Usveldt believes Sh'eenaz will take the potion to live on land, but Agloval takes it instead, wishing to no longer be beholden to his father and his kingdom, giving up his human and royal status to be with Sh'eenaz. Transforming into a merman, Agloval disappears into the sea with his new family.
Essi confesses her feelings for Geralt, spending the night together. In the morning, they realize they cannot be together as Essi wants a family and quiet life and Geralt is addicted to the road, still yearning for Yennefer. They part aways amicably, with Geralt and Jaskier leaving for Caingorn.
