The novel opens with a prologue set in 1702. The last surviving sister-witch, known as Fire, stands on the cliffs of Three Sisters Island, a refuge she and her sisters conjured during the witch persecutions. Her sisters are gone: Air was murdered by her abusive husband, and Earth destroyed herself through dark magic. Fire's husband, a silkie (a mythical seal-man), returned to the sea and forgot her. Tempted by a dark force to leap to her death, Fire instead casts a final spell: For 300 years, the island and her descendants will be protected, and three new women will rise to form a circle and face the dark. If they fail, the island sinks into the sea; if they succeed, the dark is banished forever. She leaps from the cliffs, wrapping her power around the island like a net.
Three centuries later, in May 2002, Sam Logan returns to Three Sisters Island after an eleven-year absence. A wealthy businessman and hereditary witch whose element is water, Sam has purchased the Magick Inn hotel from his parents, but his true reason for returning is Mia Devlin, the woman he left behind. Mia is a powerful witch whose element is fire who owns Café Book, the island's bookstore. When Sam enters her store, he faces hostility from Lulu, the fierce older woman who raised Mia, and from Nell Todd, the café chef and wife of Sam's childhood best friend, Sheriff Zack Todd. Mia greets Sam with cool politeness rather than the fury he expected. He reveals he has leased her yellow cottage through a shell company, and when he touches her hand, a visible spark of magic ignites between them, but she orders him not to touch her without permission.
Mia performs a blocking ritual to sever any psychic connection with Sam, but he pushes back magically, briefly glimpsing her during her private rite. On the beach, Sam apologizes, and Mia tells him plainly that he broke her heart. At dinner at Zack and Nell's home, blue light sparks when Sam and Nell's hands touch, confirming Sam's magical connection to the island through his silkie ancestry.
Sam visits Dr. MacAllister Booke, a paranormal researcher and Ripley's husband, who shares a troubling theory: Mia's task in breaking the curse may require her to let Sam go without resentment rather than accept his love. Mac confirms the curse must be broken by the autumn equinox. Sam walks to the sea cave where he and Mia first made love as teenagers and finds the Gaelic promise he carved there still glowing with magic. When he emerges, Mia is on the beach. Their confrontation escalates: Sam crushes his mouth to hers, and after she breaks free, Mia initiates her own searing kiss in return.
The dark force attacks directly. On a misty morning, Mia encounters a massive black wolf with glowing red eyes and a white pentagram scar, a mark she branded on the dark force during a previous confrontation. The wolf and a bolt of lightning nearly drive her car off the cliff road. Sam secretly adds protective charms around Mia's house. The full group gathers, and Mac explains that the dark force will target Mia specifically, preying on loneliness and loss to mirror how the original Fire sister was destroyed.
Long-suppressed tensions between Mia and Ripley, Zack's sister and a deputy who has suppressed her own magical gifts, erupt when Mia announces she intends to sleep with Sam. Their argument uncovers a painful secret: After Sam left, Ripley experienced complete empathy with Mia's heartbreak, which caused her to accidentally call down a lightning bolt that nearly killed Zack. Terrified, Ripley shut down her powers and cut herself off from Mia for a decade. The two women reconcile tearfully, with Nell joining them.
After Sam ventures into the woods alone to confront the dark force, his recklessness sparks a heated argument with Mia that leads to the resumption of their physical relationship. Sam begins a deliberate courtship with flowers, candlelight picnics, and moonlight outings. During a later intimate encounter, Sam involuntarily begins sharing his magical power with Mia. She initially refuses this merging but then opens herself and shares magic with him, an act of deep vulnerability.
At a solstice celebration, the three sisters drive the dark force back with combined power. Mia receives a vision of Evan Remington, Nell's violent ex-husband imprisoned in a mental facility, serving as a vessel for the dark force. That evening, Mia performs a dangerous astral flight, an out-of-body journey, to Remington's cell. He attacks her psychically, and she barely returns to her body. Meanwhile, the dark force lures Lulu into sleepwalking to the beach, where she nearly drowns before the others rescue her. They keep this from Mia.
Near the cave, Mia and Sam witness a vision of the original Fire sister, who tells Mia that love has no conditions and warns that time is short. Sam confesses why he left: Mia's certainty about their future felt suffocating, especially given his parents' cold, loveless marriage. Mia acknowledges her own role in holding too tight and begins to forgive him. When Sam declares his love, she responds that she cannot afford to be in love with him again.
The dark force attacks Lulu's car on the cliff road, crashing it through the guardrail. From her hilltop, Mia sprints barefoot to the scene and pours all her power into holding the teetering car on the cliff edge. Sam climbs onto the broken rail to pull Lulu from the wreckage. The dark wolf charges at Mia, who has left herself defenseless, but Nell and Ripley arrive and channel energy into her, driving the wolf away. Lulu survives with a concussion and broken arm.
As the group prepares for the final confrontation, Nell announces her pregnancy, and Ripley reveals she is also pregnant with twins. Mia quietly updates her will. Mac discovers an old text suggesting the third sister's task may involve offering her life, and the others meet secretly, alarmed that Mia may be planning to sacrifice herself.
On the night of the full moon, the dark force attacks the group as they converge on the forest clearing. Mia casts the outer circle alone, then links the others in a secondary circle representing the four elements. She steps out and runs to the cliffs, drawing the wolf after her. Standing on the cliff edge, Mia sees a circle of light in the village below: Sam has arranged for the island's residents to direct their love toward her. She absorbs this energy and unleashes her full power as Sam leaps to her side. Mia declares that she chooses love and she chooses life, clasping Sam's hand. Together they strike the dark force, which dissolves and falls screaming into the sea. The storm breaks, the fog lifts, and the curse is broken.
In the garden afterward, Mia tells Sam she loves him, not with the bordered love of the girl she was, but with a love without conditions. Sam produces two ancient silver rings, one found in the caves on Three Sisters and the other in Ireland, belonging to the original Fire sister and her silkie lover. He asks Mia to marry him, sharing a vision of her pregnant with their child. She accepts. He conjures a shower of stars from which he plucks a diamond ring, slips it on her finger, and they dance together under their own starlight.