Frank Mackey and Cassie Maddox create the undercover alias Lexie Madison together for a college drug ring investigation.

Cassie is called away from a desk job to a murder scene near Glenskehy by her partner Sam O'Neill.

Cassie examines the murder victim and discovers the dead woman looks exactly like her and was using the alias Lexie Madison.

Frank proposes Operation Mirror: Cassie will go undercover as Lexie to infiltrate Whitethorn House and identify the killer.

The murder case briefing at Dublin Castle reveals the stabbing victim was pregnant at the time of her death.

Cassie arrives at Whitethorn House in disguise and is greeted by Daniel, Abby, Rafe, and Justin on the front steps.

Cassie secretly searches Whitethorn House while the others are out and discovers a hidden journal in a bathroom panel.

FBI fingerprint analysis identifies the murder victim as May-Ruth Thibodeaux, a woman reported missing from North Carolina.

A rock is thrown through a Whitethorn window and Daniel, Rafe, and Cassie pursue and violently beat the attacker.

John Naylor is identified as the Whitethorn vandal, arrested, and brought to the station for police questioning.

Cassie meets Daniel's cousin Ned at the famine cottage and learns Lexie had been secretly selling her share of Whitethorn.

Daniel confronts Cassie in her room and demands to know who she is; she reveals her detective badge.

Frank summons all four Whitethorn roommates to Murder squad headquarters at Dublin Castle for formal police questioning.

With Daniel kept at headquarters, Rafe, Justin, and Abby recount the night Lexie was stabbed during a fight over Ned's note.

Daniel pulls the antique gun during a living room standoff, and Cassie shoots him with her concealed weapon.

After Frank Mackey discovers the murder victim bears an uncanny resemblance to Detective Cassie Maddox and was using her old alias, he proposes a bold operation: send Cassie undercover as the dead woman to identify the killer. Cassie integrates into life at Whitethorn House, living among the eccentric friend group while secretly gathering evidence. As she digs deeper, she uncovers key evidence and confronts local suspects before the investigation turns back toward the house itself. After months undercover, Frank brings the roommates in for formal questioning, and the truth of the stabbing finally begins to surface. Daniel March, the group's controlling leader, orchestrates a final confrontation that forces Cassie to use lethal force, ending the operation and his life. The case closes with the victim's many false identities stripped away to reveal the real person underneath.

The headline events of Cassie's undercover murder investigation at Whitethorn House.

Sam embraces Cassie at the murder scene and admits he feared the body he had found was her.

Sam and Cassie argue about her going undercover but reconcile before Sam leaves her flat that night.

Frank moves into Cassie's flat while they spend a week preparing her to assume Lexie's identity and lifestyle.

Cassie has her hair cut to match the murder victim's style as part of her physical transformation into Lexie.

During the first dinner at Whitethorn, the roommates reveal they told police Lexie did not have a journal.

Cassie takes her first nightly walk alone, retracing Lexie's path to the famine cottage where the body was found.

Rafe tells Cassie privately how the housemates completely fell apart emotionally and physically in the days after the stabbing.

Daniel discovers a 1919 family photo album containing a photo of William March, who looks strikingly like him.

Rafe's father phones during a poker game to offer him a job, triggering a group debate about their chosen lifestyle.

Abby privately comes to Cassie's room at night and reveals she had already guessed Lexie was pregnant.

Cassie realizes the coded letters in Lexie's journal are airport codes, revealing Lexie had been planning to flee.

Cassie and Daniel read through Simon March's family history and discover William March murdered the village girl he had impregnated.

Naylor tells detectives the full story of the changeling myth and how the March family historically oppressed Glenskehy.

Cassie finds a hidden message tin confirming Ned's deal with Lexie and plants a forged note arranging a meeting.

Sam visits Cassie after Daniel's death, repairs their strained relationship, and proposes marriage; she accepts.

Before going undercover, Cassie navigates the emotional strain of her relationship with Sam and the physical work of assuming a dead woman's identity. Once inside Whitethorn, she experiences the intimate rhythms of the housemates' lives — shared dinners, late-night conversations, and surprising confessions about each person's painful past. Small moments of connection and curiosity draw Cassie deeper into the case, from overhearing arguments to piecing together Lexie's secret correspondence. The investigation also takes shape outside the house as Cassie's research into Lexie's patterns and the history of Whitethorn begins to point toward a clear motive. Naylor's interrogation fills in the dark local mythology surrounding the manor, while Cassie's fieldwork exposes the deal Lexie had struck with Ned before her death. By the operation's end, Cassie has survived the truth of what happened the night Lexie was stabbed and accepted a new future with Sam.

The preparation, quiet discoveries, and intimate moments that shaped Cassie's time at Whitethorn.

Cassie is stabbed during Operation Vestal, loses her Murder squad position, and is reassigned to a desk job in domestic violence.

Cassie decides to accept Operation Mirror after watching a case video and hearing the murder victim's voice for the first time.

Sam agrees to Cassie going undercover only on the condition she has round-the-clock surveillance with backup ready to intervene.

Cassie calls Frank to report her first week at Whitethorn but deliberately withholds what she found in the hidden journal.

Frank reveals the May-Ruth Thibodeaux fingerprint match was itself a false identity — the real May-Ruth died in childhood.

Sam reveals to Cassie that Whitethorn House has a long history of vandalism including the phrase baby killers scrawled on the walls.

Justin privately explains the group's no-pasts rule to Cassie and reveals each housemate's painful family background.

During a nightly walk, Cassie spots a tall man in a leather jacket appearing to search for someone near the cottage.

Cassie nearly blows her cover when Justin notices she has eaten onions, a food Lexie was known to despise.

Frank reveals Daniel had legally transferred equal ownership of Whitethorn House to all five housemates, establishing a clear motive.

Daniel and Cassie kiss alone in the Whitethorn garden during a drunken house party before she stops herself.

Frank threatens to forcibly remove Cassie from Operation Mirror; she bargains him into giving her three more days.

Cassie secretly dials her former partner Rob's number and silently listens to his voice to reconnect with her real identity.

During the living room confrontation, Daniel confesses aloud for the wire that he was the one who stabbed Lexie.

Frank tracks down the murder victim's true history and reveals her real name was Grace Audrey Corrigan from Western Australia.

Cassie enters the case already fragile from the trauma of Operation Vestal, and her choice to go undercover again sets off a chain of destabilizing revelations. With each new discovery — a hidden pregnancy, false identities stacked upon false identities, a conspiracy of silence inside the house — her grip on her own identity begins to slip. Key turning points force her to choose between loyalty to her mission and loyalty to the people she has come to love. The compromises she makes, from withholding evidence to secretly calling her estranged former partner, reveal how profoundly the investigation has cost her. In a final act of self-preservation, she must face down Daniel March and find her way back to herself before the operation consumes her entirely. The truth about Lexie's real identity, revealed only at the end, brings the closure Cassie needs to finally move forward.

The decisions, discoveries, and emotional shifts that changed everything for Cassie.