Plot

Hired to probe seaside village attacks, mutant monster hunter Geralt unravels an age-old conflict between humans and sea people that threatens war between kingdoms. Aided by allies, he must solve the mystery before hostilities escalate.

Geralt is hired to kill an Allamorax which is deemed responsible for the deaths of several fishermen in Bremervoord. During the fight, Geralt is pulled into an underwater cave, where he lets the creature go after a family of sirens beg him 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 as told by him.

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 (underwater creatures) on the sirens' order. It is rumored that the king refuses to take action against the sirens because his son is in love with the siren princess, Sh'eenaz. The King and Prince orders Geralt to find and kill the ones responsible for killing the pearl divers. The King introduces a young human princess to his son, hoping he will forget about Sh'eenaz. In the meantime, the sirens are calling to their king Basim and queen Dahut for action as the humans are taking away their food. Sh'eenaz claims a marriage with Agloval could bring peace to both the worlds, but this falls on deaf ears.

Geralt, Jaskier and Essi while investigating the attack are ambushed in the morning by a Vodnik. They track and find the Vodnik's lair and confront the Vodnik into telling them who hired him in the first place, but before they can get an answer, Zelest and his troops arrive and kill the Vodnik. A battle ensues that turns into a massacre that leaves casualties on both sides. Agloval wants to marry Sh'eenaz but they argue and part ways in anger.

This exacerbates the war between the Humans and Mermaids. Because of a comment made by Jaskier and after investigating the boats, Geralt realizes the true villain is Melusina who is also a shape shifter. Melusina attacked the boats by transforming into a Kraken and was also disguised as the human princess, who he exposes in front of everyone. But, King Usveldt reveals that he had been working with Melusina in order to convince his son that Sh'eenaz is not a worthy bride. Agloval shocked by how his father had his own people killed, disowns him and leaves the throne room in shame.

While Melusina wanted to get back at her sister after King Basim chose Dahut as his queen. Melusina flees the palace after she is exposed, but Usveldt is still with the intent of war and has Geralt, Jaskier and Essi captured, along with his son Zelest, who was unaware of the treachery and decided to stand against his father. Melusina offers Sh'eenaz a potion to transform her into her lover's form. The King sails into the sea to execute the three and to kill the sirens, who fight back.

Geralt and his friends manage to escape and join the fight, during which Zelest is killed. Melusina once again transforms into a kraken and attacks everyone. Usveldt wounds Sh'eenaz and tosses her into the sea. Geralt is swallowed by Melusina but thanks to Essi delivering his sword and one of his potions. Geralt manages to cut her open from the inside and kills her with his next attack. Agloval manages to stop the fight and exposes the deceit orchestrated by the king and Melusina. Sh'eenaz is saved and healed by her mother. She 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 to his shock, Agloval takes it instead as he wished to no longer be part of the bloody dynasty created by his father. He disappears into the sea with his new family. The king blames Geralt for this, but Geralt tells him that this all happened to due the King's own ego and now he lost both of his sons as a result.

Essi confesses her feelings for Geralt and they spent 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 and still yearns for Yennefer and they part ways. Geralt and Jaskier leave for Caingorn while hoping that they don't run into Yennefer along the way. (Courtesy of Wikipedia)