The New York Times Today: "If there’s a lesson here, maybe it’s that the person she needs to rely on most is herself."

For a moment in Emma Jane Unsworth’s new book, “Animals,” it appears we’ve unwittingly blundered into some sort of millennial version of “Bright Lights, Big City.” Here is the narrator, using the second person in the first sentence, addressing us as co-conspirators from the depths of an epic hangover.

“You know how it is,” she says. “Saturday afternoon. You wake up and you can’t move.”

Not really, on both counts (luckily). But the opening both grabs us and throws us off the scent. The plot that unfurls seems simple, familiar, passed down from “Henry IV: Part II,” “Knocked Up” and many places in between: a friendship based on getting wasted and wreaking 50 kinds of havoc is threatened when one of its participants has to sober up and settle down. What makes this novel so fresh are the unexpected turns the story takes and the vividness, urgency and idiosyncratic panache of Ms. Unsworth’s writing.

The two women at the center of the book, Laura and Tyler, don’t want to grow up, though at 29 and 32, they’re not so young anymore. Supersmart and excellent company — we’re all codependent here — they’re also super-underemployed: Tyler works in a coffee shop, and Laura, the narrator and a student of Yeats, slogs away in a call center while not working on her novel, which involves a priest and a talking pig. They live together in Manchester, England.

They’re drinking and doing drugs and seducing the wrong people to anesthetize the pain of past troubles. They wrestle with dark questions of the soul — “the existentials,” they call them — and they have their own coined vocabulary. “Fonzing it” means making yourself feel better by “telling yourself that you were cool and everything was fine.” A “walk and puke” is when you stroll along and vomit without breaking stride. Laura’s description of alcohol and what it does to her is so loving and so specific that you see her need for it. “I felt my blood being exchanged for vodka and was glad,” she says.

Laura has disrupted the routine chaos of their lives by becoming engaged to a bona fide adult, a concert pianist named Jim, who has, irritatingly, gone on the wagon and whom Tyler treats with open contempt. Laura’s efforts to placate her friend and please her fiancé, to figure out what she wants and whom she wants to do it with, form the arc of the story.

But what at first seems to be a comic or possibly tragic tale about being off-your-face plastered and then having to give it up (or not) turns out to be something much more. First published in 2014 in Britain, this is the second novel by Ms. Unsworth, a young British writer full of talent, and it’s an emotionally complex and often go-for-broke-witty book about the difficulty of letting go, of making choices for yourself, of discovering that the relationships you cling to hardest may be the ones that damage you most.

Tyler is the wild friend from your 20s who inveigles you into ditching your respectable plans — a work deadline, a family dinner — to go score coke downtown with some sketchy people she just met whose names she doesn’t know. Her behavior is so aggressively undermining and outrageous that at times it feels as if Ms. Unsworth has stacked the deck against her.

But she and Laura are touchingly close. They look out for each other, understand each other. “Since meeting Tyler I’d believed that a psychic connection between human beings was possible,” Laura says. And as a pair, they can be as quick, acerbic and caustically funny as the inebriates in the classic English movie “Withnail and I,” which is saying a lot.

“What a thoroughly intolerable process,” Tyler remarks, downing most of a bottle of wine, and demanding another, on an ill-fated expedition to a boutique to help Laura buy a wedding dress. (“This isn’t a bar,” says the saleswoman.) Observing a woman at an adjoining table beat a wasp to death by repeatedly bashing it with a book, Tyler taps an ash off her cigarette and notes, “Not a Buddhist, then?”

Also, let’s face it, for humorous literary potential, it’s hard to beat a serious hangover. Like many people, I’m particularly fond of the scene in Kingsley Amis’s “Lucky Jim” when the protagonist, having burned a hole in his bedsheets after passing out drunk with a lit cigarette in his girlfriend’s parents’ house, tries to hide the evidence while assessing the damage visited upon his person.

“His mouth,” Amis writes, “had been used as a latrine by some small creature of the night.” It’s no coincidence that Ms. Unsworth has Laura quote that line after one particularly ruinous bender, as she lies in bed, the details of what she did returning to her “in shards and splinters and burning arrows.”

Things go too far, inevitably, and Tyler begins to seem less like Laura’s partner in miscreancy and more like an agent of disaster. The reader, too, begins to tire of hearing about all the various ways to get wrecked and their disastrous consequences.

Laura weighs the options. Drugs, drink and out-of-control, age-inappropriate fun on the one hand; a responsible life with a good man, but without her best friend, on the other. You’ll know where this is going, especially if you’ve seen that scene in “Trainwreck” when Amy Schumer tosses a lot of stuff into a cardboard box and gives it away.

But you’ll be wrong.

Ms. Unsworth is not one for Hallmark endings, and she’s not proposing that everyone run off and enroll in a 12-step program. She comes from a country where a little drinking to excess is not seen as an automatic moral failing. Perhaps what Laura needs is not to give up alcohol, but to give up Tyler. Or Jim. Or her writer’s block. If there’s a lesson here, maybe it’s that the person she needs to rely on most is herself.