Sirena is a mermaid, one of fifty who live in the Mediterranean Sea. Although of divine ancestry, Sirena is mortal. The only way she can achieve immortality is by making a human man love her. When the hero Philoctetes is abandoned by his fellow sailors on the island of Lemnos with a festering leg wound, Sirena cares for him until he's strong enough to survive, and the two fall in love. But Sirena wants him to love her of his own free will, not because of her enchanted voice.
Sirena contains examples of:
- Abandoned Area: Lemnos used to be inhabited, until the women rose up and killed all the men. They only left one survivor so he could warn other men away from the island. Now the women are long dead, leaving behind run-down buildings, overgrown gardens, some domesticated animals like roosters, and tools that Philoctetes uses.
- Accidental Murder: At the beginning of the book, Sirena and nine of her sisters use their enchanted voices to lure two ships to crash on the rocks near their island, Anthemöessa. The mermaids plan to seduce ten of the sailors and thereby achieve immortality. They don't realize until it's too late that only a few of them can swim. Most of the men drown, and the survivors are stranded on the tiny, barren island until they die of thirst. Sirena is sickened and horrified, and when the other mermaids come up with a plan to shipwreck men who are going to fight in The Trojan War, Sirena refuses to take part in it. Instead she swims to Lemnos to live alone.
- Character Tics: Philoctetes pulls on his beard.
- Deserted Island: Sirena and her mermaid family originally live near Anthemöessa, a small, rocky island with no plant life besides lilies and no inhabitants besides the three vultures who are the mermaids' guardian birds. Lemnos is equally empty of humans, but it's much larger and has a freshwater stream and enough plant and animal life to support Philoctetes.
- Divine Incest: Gods and fish are free to mate with relatives, but humans are not. When the mermaids were hatched, the nymph Rhodope put a curse on them forcing them to live according to the human incest taboo. If a mermaid wants a lover, her only options are men.
- First Gray Hair: Sirena has been taught how humans' hair changes color as they approach death. When she spots Philoctetes' first gray hair, she swims to Mother Dora's grotto and begs her to help Philoctetes become immortal, but Mother Dora refuses because Hera hates Philoctetes and no god would risk offending her by helping him.
- Healing Factor: After Sirena becomes immortal, she gains the ability to heal from major injuries in seconds.
- The Hermit: When Sirena reaches Lemnos, her plan is to spend the rest of her life in the waters near the island, far from any more men she might harm.
- Hot Skitty-on-Wailord Action: Sirena and her sisters were conceived when Eros seduced a parrotfish.
- Immortality Seeker: Rhodope cursed the mermaids to be mortal unless a human man should love them. For that reason, the mermaids are all desperate to find a sailor to seduce. By the end of the book, all forty-nine surviving mermaids have achieved immortality, but Sirena wishes she could give up hers so she could grow old with Philoctetes.
- I Want My Beloved to Be Happy: Philoctetes' shipmates, including Odysseus, come back after ten years to see if he's still alive. They want to bring him to Troy because a soothsayer predicted he could help win the war. Sirena realizes that if he goes he will never want to return to Lemnos. But the nymphs tell Sirena that Philoctetes' sense of honor demands he go, and Mother Dora also predicts that a human doctor will be able to permanently cure the wound in his leg. Philoctetes tells Sirena he'll stay if she asks him to, but Sirena tells him to go.
- Magic Music: To make up for Rhodope's curses, Mother Dora blessed the mermaids with magical voices that can make men instantly fall in love with them. Sirena sees it as a very mixed blessing, and during her romance with Philoctetes she regrets letting him hear her sing because she wanted him to fall in love with her, not with her voice.
- Mayfly–December Romance: Sirena knows that she'll love Philoctetes no matter even in his old age, but she dreads the day of his death. In the end it doesn't matter, because he leaves the island before he reaches middle age.
- Music Soothes the Savage Beast: Sirena drags herself on land and finds herself face to face with a bear. It walks towards her, growling, but before it can attack, Sirena starts singing. The bear stands still and listens to her until Philoctetes chases it away with his arrows.
- Not Brainwashed: A positive example. For years Sirena assumes Philoctetes only loves her because of her voice, but he eventually tells her that he realized he loved her before he heard her sing, and her voice had nothing to do with it.
- Rescue Romance: During Philoctetes' first few days on the island, Sirena brings him fish she caught, fruits and nuts she picked from abandoned groves, and fresh water in a bowl she found. After he recovers from his sickness, she continues to tend to his wound.
- Revenge by Proxy: Philoctetes was bitten by a snake sent by Hera, who hated him for being friends with Heracles and still wants to kill him even though Heracles is dead. The other sailors leave him on Lemnos because if they harbor someone who is hated by Hera, they're afraid they'll become victims of this trope as well.
- Underwater Ruins: Early in the book, Sirena swims over the ruins of a town that was flooded by an earthquake.
- Woman Scorned: The nymph Oenone, whom Paris abandoned for Helen, asks the mermaids not to sink any of the Greek ships because she wants all the soldiers to go to Troy and kill Paris.
- Wound Licking: At first Sirena has no idea how to treat Philoctetes' leg injury, so she imitates the land animals she's seen and licks it. The pressure causes some of the pus to leak out. She pushes the rest of the pus out with her hands and bathes the wound in seawater.
- Wound That Will Not Heal: Philoctetes' wound never heals during all his years on Lemnos. Every morning Sirena drains and cleans it, and he's fine for the rest of the day, but he wakes up the next morning with the wound painful and festering again.
