At first Mally Trenglos views Barty Gunliffe as her antagonist, her enemy as, having trenched on her preserves in Malachi's Cove, he competes with her in gathering seaweed to sell as fertilizer. He, in turn just laughs when she swears that she will gather it where he cannot, and he calls her a mermaid because her hair hangs like seaweed. Angered, Mally retorts,
"Mermaid, indeed! I wouldn't be a man to come and rob a poor girl and an old cripple. But you're no man, Barty Gunliffe! You're not half a man."
Barty only encroaches on her area because she is so bellicose; were she to speak civilly to him, he would get his father to pay her grandfather for the use of the path to the cove. But Mally will not relent, even threatening not to "lift a hand to help him out" if he comes down when the wind blows into the shore.
One day as they are tensely competing against each other, "Mally's hook was better than his fork, and Mally's skill was better than his strength." And so, angry that she is able to plunder more seawood than he, Barty sees a large treasure of seaweed thrown about on the rocks near a whirlpool, named Poulandioul by Mally--"the hole of the Evil One." Nonertheless, Barty is tempted to gather this treasure despite the danger.
It is when he falls into the whirlpool that Mally forgets her hatred for the young man; instead, she risks her life to save him after his head is dashed against a rock. Stretching her hook that she uses to catch seaweed, she calls to him to grab it while she seizes his collar and pulls. Then, the sea pushes him onto a ledge; as he lies there, Mally pushes his hair from his bleeding forehead and falls in love:"...as she looked at him she knew that he was beautiful."
Finally, she is able to move Barty, amazed at her own strength. Soon, her grandfather meets them as he has observed the incident. Once at the top of the cliff, Matty runs to the farm of the Gunliffe's where she asks for Mr. Gunliffe, telling Mrs. Gunliffe should send for the doctor. When Mr. Gunliffe, a rather stern man, especially when angered, appears, he and his wife follow Mally. After the old man says something to Mr. Gunliffe, he grabs Mally,
"If he has come by his death between you, your blood shall be taken for his...."
Ironically, he suspects Mally of having taken Barty's life, but, in truth, she has nearly lost her own in saving him. And, when they find their son, pale with a bloody gash upon his forehead, Gunliffe vows, "They shall give us blood for blood." For, it is not until Barty regains consciousness and asks for the girl that the Gunliffe's realize that she has saved their son, saved him that she could love him, too.
The following day, Mr. Gunliffe comes for Mally and takes her to see Barty. Gunliffe holds Mally's hand because he knows now that she has saved his son, and that
...he had injured her instead of thanking her. He was now taking her to his heart, and as words were wanting to him, he was showing his love after this silent fashion.
Mally asks him nothing; she understands his gratitude, and after she visits Barty, who kisses her hand as she finds him "like an angel," Mally is given tea and hot cake with the Gunliffe's; afterwards, "she began to think that the Gunliffes were good people--very good people" because Barty's father expresses his gratitude and Mrs. Gunliffe tells her, "Mally, thou art my child now, and I shall think of thee so."
Hence, Mally has gone from loathing Barty to loving him, while Mr. and Mrs. Gunliffe have changed from accusing Mally of having tried to drown their son to being very grateful and loving toward the girl-mermaid to whom their son gives his love and marries.