Four or five years earlier, Dublin detective Cassie Maddox went undercover to expose a network of drug dealers. Posing as a graduate student at University College Dublin, she used the alias Lexie Madison.

Now, as "The Likeness" opens, Cassie is contentedly working for Dublin's Domestic Violence Squad when she receives an early morning phone call from her lover, the comely Detective Inspector Sam O'Neill. His voice sounds strained as he insists she meet him at a murder scene in the tiny hamlet of Glenskehy. To Cassie's astonishment, the stabbing victim, sprawled on the floor of a long-abandoned "famine cottage," is her exact double. Moreover, a Trinity College ID card found on the body reveals the deceased to be none other than Lexie Madison.

In her present confusion Cassie muses: "We made Lexie Madison bone by bone and fiber by fiber, we baptized her, and for a few months we gave her a face and a body, and when we threw her away, she wanted more."

It's a pivotal scene, made all the more compelling because of author Tana French's decision to have it narrated in the first person. Cassie's consistent, single-consciousness voice, with its warm and confidential immediacy, is perhaps the novel's chief asset. Nothing is lost on Cassie, who has an "instinct for the truth." Her flippant, often funny remarks never lose their punch. And the protagonist as narrator makes for a pleasing unity, allowing the story to glide seamlessly between actions and reflections.

From the start Sam, Cassie and her former undercover boss, detective Frank Mackey, face a double challenge: Who killed the young woman? And who is she? Frank persuades Cassie that the quickest way to solve the case is to spread the word that Lexie's stab wound wasn't fatal. Cassie, posing as Lexie, will simply take up Lexie's life, impersonating her until the murderer is apprehended.

So Cassie "returns" to the antiquated Whitethorn House deep in the woods outside Glenskehy, where Lexie lived for the previous few years. Like Lexie, her four housemates are graduate students at Trinity College in Dublin, and all are well-rounded characters endowed with serious ideas. Taking an instant liking to them, Cassie settles in comfortably: "I had slid into her like sliding into comfy old jeans," she notes.

The routine includes the thoughtfully prepared, formal evening meal; high-class, witty but wise repartee; rowdy, late-night poker games, and companionable house-improvement projects. Yet she senses a vague but menacing undercurrent; clearly, a sinister cover-up lurks beneath the fun. Soon she's torn between loyalty to her undercover duty and her new friends. "Whatever had been going on here, I needed to put my finger on it before Frank did," she reflects.

Tana French trained as an actor in college and now works mainly in theater, which explains her perfect sense of dramatic timing, and she cleverly unfolds her plot while conveying the mounting tension at White- thorn. Ominously, in a conversation with housemate Rafe, Lexie/Cassie learns that Daniel, another housemate, first believed that Lexie had died. Rafe tells Lexie/Cassie: "Daniel had some bizarre, convoluted theory about the cops claiming you were alive just to mess with people's heads."

Regrettably, the last third of the novel runs aground, and the denouement is gratuitously prolonged. Still, the novel is exceptional, and readers with an affinity for whodunits authentically set in Ireland will not be disappointed.

Katherine Bailey also reviews for the Philadelphia Inquirer.