US couple, Roy and Jessie, embarks on an adventurous journey aboard the Trans-Siberian train from Beijing towards Moscow. Aboard, they encounter fellow travelers, including a Spanish man named Carlos (Eduardo Noriega) and his girlfriend Abby (Kate Mara), both of whom leave quite an impression on them. Intrigued, Carlos shares with Jessie a selection of "rare" souvenir Matryoshka dolls that he is proudly carrying. However, during a moment of exploration in Irkutsk, Roy inadvertently misses the train, forcing Jessie to disembark and await his return on another train. Carlos and Abby also choose to exit the journey and head to a nearby hotel.
While at the restaurant, Jessie spots dolls that are strikingly similar to Carlos' collection, causing a strained altercation when Abby expresses jealousy over Carlos's interaction with Jessie. The next day, Carlos visits Jessie under the pretense of needing to use her bathroom, leaving her alone momentarily in their shared accommodation. This solitude is interrupted by a phone call informing Jessie that Roy intends to rejoin her later that day.
However, tensions escalate when Carlos makes an unwelcome advance on Jessie, leading her to defend herself fiercely. In a surprising turn of events, the encounter results in Carlos's death, compelling Jessie to return to the hotel and subsequently rush to the railway station to find Roy. Meanwhile, a dark twist unfolds as Ilya Grinko, a Russian narcotics officer, becomes Roy and Jessie's new roommate aboard the train. Jessie discovers the stolen dolls in her own luggage, leading her to a shocking revelation that Carlos was smuggling heroin inside them. Ilya, unfortunately, has ulterior motives tied to a powerful Russian drug lord and is in pursuit of both Carlos and Abby.
As the narrative unfolds, Jessie senses danger in her possession of the dolls and ultimately confesses to Roy and Ilya about the truth behind them. The following morning brings a chilling realization as Jessie and Roy wake up to find the majority of train passengers missing, leaving only Ilya and his partner Kolzak on board. They abruptly halt the train in a desolate area, where they lead Jessie and Roy to an abandoned military bunker, discovering Abby being tortured inside. Ilya reveals a more insidious plot, implying Abby's guilt in heinous crimes, casting doubt on Jessie's previous perceptions of innocence.
Faced with this chaotic reality, Jessie and Roy manage a daring escape by commandeering the train, but their peril escalates when disaster strikes in the form of a head-on collision with another train. Chaos ensues as soldiers board the train, leading Ilya to eliminate Kolzak to preserve his cover as a law-abiding investigator rescuing Jessie and Roy from alleged captivity. In a moment of concealed panic, Jessie struggles to convey the truth to the Russians, failing as she slips into unconsciousness and witnesses Ilya disappearing into the forest.
Following their harrowing experience, U.S. officials inform Jessie and Roy of Carlos's concerning past, revealing his criminal history filled with theft and assault. The officials have little knowledge regarding Abby, labeling her as just another young woman ensnared by a dangerous partner, yet Jessie remains silent about her own involvement in Carlos's death, hinting that Roy might have heard her confession just before the train catastrophe.
Amid their stay in Moscow, an unsettling encounter unfolds as Jessie sees a billboard depicting a girl by a dock, reminiscent of Abby's dreams. Compelled by this connection, Jessie insists on reaching out to Abby in the hospital, ultimately revealing to her the grim fate of Carlos.
In a poignant closing scene, Abby, now limping from her trauma, stumbles upon Carlos's lifeless body in the snowy aftermath by an old church. A mix of emotions swirls as she discovers a hidden stash of stolen cash sewn into his jacket, prompting her to take it without a moment's hesitation, walking away from his body without a backward glance.
