Three Sisters Island off the Massachusetts coast carries a dark legacy: in 1692, three witches sacrificed themselves to save the island, binding their descendants to a bargain that must one day be fulfilled. Nell Channing arrives as a fugitive from an abusive husband, discovering not only safety but her own magical heritage and a connection to the island's mystery.

Nell is cautious and deeply wounded when she arrives on the island, carefully rebuilding herself beneath an assumed identity. Mia is confident, magical, and guarded after an old heartbreak. Ripley is a skeptic and deputy sheriff who resists her power even as it demands to be used. Each woman is fully realized and deeply relatable.

Across three books, Nell, Mia, and Ripley each find love and claim their elemental power as the descendants of the original Three Sisters. Together they must face the reawakening threat of the bargain's terms and the dark force that wants the island's power, culminating in a final magical battle that determines the island's fate.