Anna Matheson chartered the privately run Philadelphia Phoenix for a nonstop overnight run to Chicago and lured six people she blamed for destroying her family—Judd Dodge, Herb Pulaski, Sally Lawrence, Edith Gerhardt, and Lt. Col. Jack Lapsford, plus the intended Kenneth Wentworth—onto an emptied train. She revealed the 1942 sabotage that killed 37 soldiers including her brother Tommy, the frame-up and prison murder of her father Arthur, and her mother Margaret’s suicide, declaring she had sent irrefutable proof to the FBI in Chicago. A “wrinkle” emerged when Dante Wentworth arrived in place of his father, and the train sped past all stations as planned despite the group’s attempt to pull the emergency brake.

Confrontations escalated: Judd discovered the train seemingly empty; the group argued over culpability and Anna’s mailed evidence; and when Lapsford yanked the brake to no effect, the party pushed forward to the locked cab. Dante, confirming Anna’s control, ordered the engineer to continue. Back in the lounge, Dante mixed martinis; Lapsford faked a heart attack, and moments later Judd appeared to die, apparently poisoned. With suspicion mounting, Anna led a search and soon uncovered a box of rat poison in the galley. She convened everyone, but tensions sharpened when Edith privately reported seeing Lapsford near the dining car and then was found dead, staged with a curtain cord though likely smothered with a cushion. The ninth passenger—Reginald “Reggie” Davis—revealed himself as an FBI agent sent to ensure the six reached Chicago after the bureau received Anna’s evidence.

Interrogations followed as a blizzard worsened. Herb, wracked by debt and fear, claimed he had stayed in his room; Lapsford stonewalled and was accused of lying about the galley; and Sally confessed Kenneth had entrapped and blackmailed her into forging evidence after Tommy’s funeral. Anna found Judd’s “poison” likely came from the train, and later realized Judd never drank his martini; he had used rat poison residue and staged foam to fake death. When Anna, Reggie, and Seamus checked Judd’s room, he was gone; soon after, Seamus was stabbed in the galley—seemingly by Judd. But the timeline twisted again when Seamus led Anna to Edith’s compartment where Judd’s freshly strangled body was stuffed in the bathroom, proving he had been alive and then killed.

As fear grew, Herb tried to force a stop by taking Anna hostage in the dining car; she fought him off with a concealed knife and training from Aunt Retta’s arranged lessons. Dante tended Anna’s wound and admitted he had anonymously supplied Anna’s aunt with incriminating materials from Kenneth’s safe, including letters revealing Margaret had once been engaged to Kenneth. Anna confronted Sally, who disclosed her coerced role via a honey trap and suggested Seamus might be the killer. After Anna reaffirmed trust in Seamus, she, Seamus, and Reggie discovered Herb murdered behind a locked door, throat slit, with a planted trail implicating Anna: her father’s pin on the floor and, later, her room’s window open and a curtain cord missing. Anna argued she was being framed through methods tailored to her—poison after her accusation, a staged cord after she grabbed Edith’s throat, and a slashed throat after her knife was revealed—and deduced the killer could traverse the exterior.

Reggie gathered the survivors to hunt Judd, now suspected alive and violent. During the search, Reggie was found stabbed in the galley and Anna stitched him up with a sewing kit and vodka. After a brief collapse and recovery, Anna, Seamus, and Dante discovered Judd’s corpse and concluded another killer was at work. Lapsford then suffered a real heart attack; Seamus saved him with a pill from his box of muscle relaxants while Anna forced partial admissions. When Seamus later discovered his gun missing, two shots rang out; Anna found Sal apparently dead with a hole in her forehead and Seamus’s gun on the floor. Convinced Dante was the killer, Anna had Seamus tie him up, but she then staged Dante’s restraint as a ruse: she lay in wait, heard someone slip into Lapsford’s room, burst in, and tackled the attacker mid-suffocation—Seamus—who apologized and admitted she was not supposed to find out.

In a private reckoning, Seamus confessed to killing Judd for vengeance, returned Anna’s father’s pin, and fled to the baggage car. He handed Anna a revolver, told her she knew how to end it, and stepped off the moving train. Grieving but analyzing his wording, Anna realized Seamus had only admitted Judd’s murder; Edith’s smothering, Herb’s throat-slitting, and Sal’s “shooting” pointed to another killer. As her screams echoed down the cars, Dante freed himself, Lapsford schemed an escape, and Sal—whose death had been faked with lipstick and nail polish as part of Anna’s earlier plan to draw out Seamus and steal his gun—opened her eyes.

Before dawn, Anna anticipated a final attempt on Lapsford and confronted the intruder in his room: Agent Reggie Davis. He confessed relief at exposure, revealed his vendetta over his father’s death on a troop train, and detailed coercing Judd to help kill the witnesses, staging Edith’s scene with Anna’s drapery cord, slitting Herb’s throat via a window escape that ruined his shirt, stealing Anna’s pin, and self-inflicting his stab wound to misdirect suspicion. He tried to recruit Anna; she stalled until Sal knocked him out with a champagne bottle. Anna then armed Dante with Seamus’s gun to guard Sal and Lapsford, loaded bullets she found in Reggie’s room into Reggie’s gun, but Reggie escaped out a window. Anna pursued him onto the roof through the blizzard, forced him back over the observation car skylight, fired into the glass, and both crashed into the car, where she regained the gun and prepared a nonlethal shot just as the Phoenix stopped outside Chicago.

Reggie fled through the cars; Anna fought him in the club car and injured him with kicks and a hurled cup, then fled to the baggage car seeking the engineer. The reinforced door opened not to Burt Chapman but to Kenneth Wentworth, who had secretly replaced the crew in Philadelphia and driven the train himself. In the locomotive, Kenneth taunted Anna until she pressed him about Margaret and motive; he revealed Tommy was his biological son and admitted ordering Arthur’s prison murder. With Reggie present urging vengeance, Anna forced Kenneth to confess he had destroyed her family and others, then, remembering Seamus’s charge to end it, shot Kenneth Wentworth as the long, nonstop journey reached its reckoning.