In the sultry ambiance of Venice, Italy, in 1947, we find the restless Hercule Poirot (Kenneth Branagh) grappling with the haunting shadows of his past. After retiring from his life as a detective, Poirot resides in this mesmerizing City of Water, yet the persistent whispers of unresolved mysteries and pleas for assistance taunt him, preventing him from finding any semblance of peace. At his side is Vitale Portfoglio (Riccardo Scamarcio), a former police officer turned loyal bodyguard, who fervently shields Poirot from the unrelenting calls for help.
One fortuitous evening, Ariadne Oliver (Tina Fey), an old friend and established author, arrives at Poirot's doorstep, her spirit dampened by the lackluster reception of her recent literary works. In a bid to uplift both their spirits, she extends an invitation to a Halloween soiree at the luxurious palazzo of Rowena Drake (Kelly Reilly), an ex-opera singer. The highlight of the evening promises to be a séance conducted by the enigmatic Joyce Reynolds (Michelle Yeoh), known in the press as "The Unholy Mrs. Reynolds."
Navigating through the winding canals, Poirot and Vitale join Ariadne and soon arrive at the palazzo, where children are mesmerized by eerie stories of the building’s tormented history. According to local legend, the palazzo was once an orphanage haunted by the tragic echoes of the Plague, birthing a curse known as the Children's Vendetta. Any person marked within this realm is said to be pursued by the vengeful spirits of children wronged in life. Among the guests is a young boy named Leopold (Jude Hill), who remains absorbed in the works of Edgar Allan Poe, favoring solitude over the raucous gathering. Meanwhile, the air is thick with sorrow as Rowena mourns the recent death of her daughter, Alicia (Rowan Robinson), leaving her struggling with profound grief.
In this chaotic ambiance, Olga Seminoff (Camille Cottin), the housemaid, senses Leopold's unease as he longs for his father, Dr. Leslie Ferrier (Jamie Dornan), who remains consumed by the loss of Alicia. The evening further escalates with the arrival of Joyce Reynolds and her assistant, Desdemona Holland (Emma Laird), who introduce an air of mystique. However, the situation intensifies when Maxime Gerard ([Kyle Allen]) unexpectedly surfaces, claiming to have been summoned by another, igniting tension among the grieving family members.
As they delve into the séance, Joyce takes center stage, her eerie presence creating a sense of dread among the attendees. Olga reveals to Poirot that Alicia experienced eerie visions before her demise, and as Joyce appears to channel Alicia's spirit, Poirot’s keen observation soon unveils a hidden accomplice, Nicholas ([Ali Khan]), secretly manipulating events from a concealed chimney. The atmosphere grows increasingly unsettling as Joyce displays bizarre behavior, reminiscent of Alicia's torment.
Despite the disquiet, Poirot attempts to exit but is held back by Ariadne, prompting a deep conversation with Joyce, who reveals her fear of being a mere pretender. She encourages the detective to lighten his emotional burden, as if she can sense the weight of his own tragic past.
As the night darkens, a sudden attack leaves Poirot stunned and gasping for air, only to be revived by Vitale. Just then, a blood-curdling scream disrupts the gathering as Joyce's lifeless body is discovered, grotesquely impaled. With a tempest roaring outside and the floodwaters rising, the guests find themselves trapped within the palazzo's walls, stoked by a rising tide of panic and paranoia regarding the malevolent force that may have taken Joyce's life. Poirot, an astute detective, locks all exits, igniting his investigative instincts as he teams up with Ariadne to sift through narratives and unveil the truth.
Their inquiry leads them to rediscover a lost stuffed rabbit, once beloved by Alicia, which ignites Poirot's curiosity and methodical approach. Moments of insight come to him while reflecting in the restroom, where he believes he sees Alicia's ghost in the mirror's surface. In a series of interviews, he gauges the motives and hidden pasts of Nicholas and Desdemona, revealing their history as Romani refugees and the undercurrents of petty theft that may connect them to their recent misfortune.
Suddenly, a phone call adds to the unease, bringing forth a harrowing silence before Poirot glimpses the figure of one of the ghostly children. As Ariadne's timely appearance distracts him, the haunting sounds of a child’s song echo through the palazzo, leading Poirot to believe that the source lies in the basement—only to be dismissed by Rowena. Driven by curiosity, the group descends, uncovering a hidden chamber marked by the cryptic word "dottore," a direct connection to the painful history of the Children's Vendetta.
Amidst the simmering tensions, Dr. Ferrier's anxiety culminates in a tumultuous confrontation with Maxime, quelled only by Leopold's intervention. As events unfold quickly, Poirot observes details about the fragile relationships between the guests while investigating Maxime's claim about the life-altering bond he shared with Alicia, one crafted amid love and maternal disapproval.
Tracking down clues tied to Maxime, Poirot deciphers a message tied to an apple, escalating his curiosity further. Just as Ariadne arrives bearing an apple—potentially crucial to the case—truths unfold revealing the intricacies of the group’s interconnections. The implications of betrayal intertwine as Poirot confronts the evident conspiracies woven into their narratives.
However, chaos ensues as Ferrier's body is later discovered, a cruel twist of fate bereft of hope for both him and Leopold. With the storm’s rage echoing outside, Olga shares revelations about Rowena's consistent devotion to Alicia, tinged with anxiety over the whispers from beyond that plagued the household—an omen of treachery.
As Poirot examines memories, he stumbles upon poignant images of a young Alicia, stirring echoes of familiarity while uncovering a flower hidden within a cherished photograph. An accidental act by a parrot draws his attention to the remaining fragment of mystery, and the detective’s mind races, connecting the dots into a coherent narrative.
Ultimately, Poirot exposes the shocking reality: Rowena herself is behind the gruesome acts. Her elaborate scheme, masked by her grief, aligns to create illusions of guilt, driven by jealousy and the need to control.
As Poirot's confrontation reveals the dark depths of revenge and manipulation—Rowena, consumed by her own motives, resorts to a desperate escape. The tragic allure of the water beckons her, drawing a chilling parallel to Alicia's earlier death and leaving her fate sealed in a watery grave.
In the aftermath of anguish and revelation, Poirot and Ariadne share a bittersweet farewell before he sets off on a new, perhaps unending, investigation, encountering a sorrowful stranger whose tale demands his extraordinary acumen to unravel yet another myriad of deceit and tragedy.
