Is it love at first sight, or a gamble of fate? Captain James Harlow, third son of a viscount, expects nothing from life and yearns for adventure. He gets his wish when he's overcome by a group of young miscreants with ties to the peerage, losing his hand in the encounter…and nearly his life. If not for a mysterious mermaid who emerges from the depths to save him. Ione isn't like the other Nereids. Her scales and hair aren't as bold in color, and she has no desire to drown the mortals she mates with. So when a handsome mortal is left to die in the sea before her eyes, she can't stop herself from saving him—and trading her fins for legs to follow him on land. With only three days to determine if they belong together, the pair are caught in the midst of a sociopath's schemes to tear James' life asunder. But can the young earl be stopped before he harms anyone else?

Bryce Stirling, the Earl of Winterthorne, has no desire to travel to the English countryside for his cousin Ione's wedding to a viscount's son. Yet, when he takes refuge in the Summerfield greenhouse to escape socializing with the wedding guests, he is instantly captivated by the beautiful woman who has chosen the same location to hide away from her family—and a persistent triton who wants her for himself. Miss Wendelin Harlow has three older, and far too over-protective, brothers. When the seductive Lord Shellington takes an interest in her at the wedding, her brothers swoop in, and Wendelin seeks the comfort of her greenhouse to avoid the lot of them. When she meets the Earl of Winterthorne, she wishes to learn more about the mysterious man who is frequently discussed in ton circles, but rarely seen. It doesn't take long for the two immortals to learn they desire the same woman, and Bryce somehow allows Lord Shellington to talk him into a wager for Miss Harlow's love. The winner gets the woman, but the loser gives up their immortality.