In “Mermaids in Paradise” that discussion is about the different ways people see the world, and how perceptions form belief. Chip, an earnest jock, idealizes “Middle Americans,” their love of cruises, their children’s homemade toys, their knowledge of grain production. He spends his free time playing video games. Deb’s best friend, Gina, on the other hand, is a self-­described “failed academic” (who calls this phrase redundant), so full of irony that “everything’s performance art with her.” Chip and Gina represent opposite ways of seeing the world, and Deb, influenced by both, finds herself somewhere in between.

Deb’s humor, empathy and insight make this story disarmingly funny, but “Mermaids in Paradise” is also an ambitious exploration of belief. When Chip first reports that he’s seen the mermaids, Deb feels separated from him, lonely. “I wanted the old Chip back, the Chip for whom there was real life on the one hand, without mythic creatures, and video games solidly on the other hand, where mythic creatures cavorted quite abundantly.” She gropes for logical explanations — LSD in the breathing tubes of their diving gear, perhaps. But eventually, even before she sees the mermaids for herself, she reverses her stance, comparing her change of heart to a political candidate’s “flip-flop on abortion when the demographics called for it.” She decides to believe Chip, reasoning that if the mermaids are real, “so much the better for us all,” and if they’re not, it’s no personal threat to her. Millet suggests that belief might, in the end, be more arbitrary than we would like to think, simply the consequence of living with and loving the people we do.

“Each country has its own hysterics,” Chip likes to say, and while Deb and Chip are baffled by Middle America’s creationists, people “suspicious of biology and mortally offended by an ape,” the novel is not at all a simple indictment of faith. Rather, the mermaids, in their brief, magnetizing appearances, suggest the vitality of things beyond rational explanation. “I saw her face,” Deb says, “I saw it full-on — not mythically beautiful, not mythically homely, just a face, its skin sickly white in the water. I saw gills on her neck, their slits opening. I saw a look of surprise.”

As much as the novel examines the dangers of belief, it also interrogates irony. When Deb is kidnapped, she texts Gina: “Locked in a room in our resort. Chip coming to save me (hope). Saw real mermaids. Even got video, but video stolen.” Gina thinks it’s a joke (“Her irony was far too bulletproof for a mermaid sighting; her irony was a Kevlar of the mind”). The only way for Deb to signal that she’s serious is to text, “On the life of your mother,” their agreed-upon message. When they were in high school, Gina’s mother died of cancer, and the girls came up with the code to indicate when they were being “hard core,” not joking. And suddenly, the novel shifts emotional registers: In the middle of wondering how (or if) Deb will escape, we’re contemplating the role of irony as a shield that can distort emotions and facts, and at the same time, the pointedly unironic heart of this friendship.

A handful of small illustrations appear throughout the text. Like casual iPhone snapshots, they evoke blurry photographs — a look over Chip’s shoulder at a computer screen, people watching a video of the mermaids — often with a thumb or a finger within the frame. The pictures are seductive, not as “evidence” so much as small staged fictions themselves.