In Sirena, Napoli weaves the legends of the Trojan War with the myths of the Sirens and centuries-old lore about mermaids into a beautiful story of young love, with all its wonders and fluctuations. Sirena is one of ten mermaid sisters inhabiting the waters of the Ancient World. The school of ten fish-tailed beauties is matched by another four groups with ten young mermaids, just turned seventeen. Their goal in life is to attain immortality by having a human man fall in love with them. It is to this end that they have learned their song, given to them by Mother Dora, wife of Nerius, daughter of Oceanus, when she saved them from a certain death. They would have been swallowed by the nymph Rodophe after Eros had sexual relations with their mother, the parrot fish, Iris, kept as a pet by Rodophe.

When a ship is sighted nearing their rocks, the sisters sing sweet and clear, causing the ship to be tossed onto their rocks, throwing the sailors, many of whom cannot swim, into the sea. Many men are drowned, and the others swim to an island, where they starve on the barren shore, whose only vegetation consists of beautiful but inedible plants.

Sirena sees her siblings cursed by the men. She swims away in confusion. If a mermaid's immortality must be bought at the price of a human's death Sirena cannot bear to fulfill her destiny. She swims to the isle of Lemnos, long ago deserted when its women killed its men and put their king adrift at sea. It is there that fate allows her to watch as a ship puts into harbor and men abandon one of their number on shore.

On Lemnos, Sirena first sees Philoctetes, protege and inheritor of the quiver and arrows of Heracles. He has been marooned and left to die on the island by his companions after suffering a leg wound caused by a serpent sent by the goddess Hera, who hates Philoctetes because of his love for Heracles, illegitimate son of her husband, Zeus.

Sirena is attracted to Philoctetes by his beauty, his humanity, and his sense of humor. She immediately falls in love with him and determines to save him from a poisonous death. She wants him to love her, not for the gift of immortality that his love will bring her, but for herself. She vows never to use her song to ensnare his love. Philoctetes in turn is fascinated by the mermaid. It-is she who saved his life by her ministrations. Her beauty, naivete, and intelligence cheer him. He delights in her ability to listen to his stories and to challenge them with her own version of the tales.

The young Philoctetes and Sirena become friends. Although she has vowed never to use song to entice him, Philoctetes nevertheless hears her when she sings to an attacking bear in order to save her own life. When he understands that his love for her may be part of an enchantment, Philoctetes reassures Sirena that "we are all of little pieces . . . all part of this and that . . . air, water, fire and dirt, but she is more, much more . . . she is Sirena." With his avowal, their love is consummated.

Napoli uses in Sirena, as she did in The Song of the Magdalene, The Magic Circle, and Zel, the powerful suggestion of supposition. In other words, she asks her readers to wonder "what if?" Her ability to take fairy tales, Biblical accounts, and, in this case, myth and transform it with an entirely new approach is beyond peer. Literate readers know of the Sirens, women-fish hybrids, who lured sailors to their death, and the stories of the mermaids of Hans Christian Andersen. In Sirena, Napoli's mermaid has a soul, and so her story is new.

Sirena's idyllic life with Philoctetes is always threatened by the fear that his love may be the result of his enchantment. His removal of himself to the inside of the island—to the remains of the city or to his cave high on a hill—speak to the difference in their beings. In their love, the young couple strive to make each other happy. She watches him build a boat and helps him launch it, though she is delighted when it sinks and they must return home. He restores a temple and lines it with the mosaics she once glimpsed in wonder, but stronger forces threaten their love. Sirena is called by the biorhythms of the sea. She leaves the island periodically to swim with the porpoises as they push forth new life—fulfilling their destiny in the water world. Philoctetes needs to build and harbor the hope to fight as a warrior once more. His arms ache to hold the bow of Heracles—to avenge Heracles' death by vanquishing Paris. Neither of the young lovers can give voice to their deepest desires.

It is a small discovery that sets the story spinning towards its inexorable end. Sirena notices a gray hair in Philoctetes' head. This discovery makes her leave Philoctetes and race to Mother Dora in an effort to buy his immortality with her love. She even begs help from Oenore, the sea nymph Paris had loved before he left her for Helen—finally offering her own immortality to share life with Philoctetes as long as he might live.

But the gods have already decreed their future. Despite true love and their years of happiness together, not even Napoli's skilled storytelling can save Paris from Philoctetes' arrow. And so the lovers must part—each to take part in the settings history and myth have decreed for them—leaving the reader with a tear and a sigh.