As the alarming discovery of a pneumonic plague strain at the U.S. mission within the International Health Organization comes to light, a trio of terrorists hatches a plan to unleash chaos across America. Two of these malicious figures are taken down by security forces, with one even dying despite heroic rescue attempts. However, the last remaining terrorist manages to escape and ends up hospitalized, revealing his Swedish identity. This prompts an intense exchange between Dr. Elena Stradner and Colonel Stephen Mackenzie regarding the true nature of the plague strain. Stradner fears it could be a biological weapon of mass destruction, while Mackenzie argues it's merely a strain meant for disposal.
Meanwhile, the third conspirator, Eklund, cleverly evades capture by hiding on a train traveling from Geneva to Stockholm. As Stradner pushes for the authorities to intercept this potential threat, Mackenzie voices his fears about the risk of infecting the train’s passengers. He proposes rerouting the train to an abandoned railway that leads to Janov, a former Nazi concentration camp, where the travelers could be adequately isolated. This plan, however, poses a perilous risk as it crosses the Kasundruv Bridge, an unstable steel arch structure that has been out of service since 1948.
Mackenzie is acutely aware that traversing such a treacherous route could bring about disaster, heightening the stakes for the passengers, which include a diverse array of individuals: the acclaimed neurologist Jonathan Chamberlain, accompanied by his ex-wife Jennifer Rispoli Chamberlain, a writer; Herman Kaplan, a Holocaust survivor; and Nicole Dressler, the wife of a German arms dealer involved in an affair with Robby Navarro, a young drug trafficker pursued by Haley, an undercover Interpol agent masquerading as a priest.
As Chamberlain learns about Eklund's presence on the train from Mackenzie, he is also informed of a caged basset hound inadvertently infected with the plague found among the bags. In a desperate attempt to contain the spreading disease, Mackenzie attempts a helicopter evacuation of Eklund, which ultimately fails. As the train nears a tunnel, the potential carrier dog must be hurriedly transferred to the Institute of Health Organization's laboratory, where Dr. Stradner will oversee its analysis.
Chamberlain is faced with the grim realization that the plague's mortality rate stands at an alarming 60%. Meanwhile, Mackenzie, seemingly disconnected from the dire circumstances, fabricates a story about anarchist bombs along the rail line, declaring that the train will now be redirected to Nuremberg, sealing it within an oxygen supply system and stranding it with a U.S. Army medical team. The lifeless body of the deceased terrorist lies in a hermetically sealed coffin, lingering ominously over the unfolding crisis.
Chamberlain grows increasingly uneasy as he comes to a disturbing conclusion: the disease might not be as lethal as originally feared, as only a few passengers have actually fallen ill. Taking the initiative, he radios Mackenzie to propose isolating the infected section from the rest of the train. However, Mackenzie remains rigid, adhering to orders that obscure the truth about covert biological operations conducted by American military forces in neutral territories — a scandal set to be buried beneath the rubble of the Cassandra Crossing.
As tensions heighten, Chamberlain joins forces with Haley to seize control of the train, knowing that failure to act could lead to a disastrous finale at the bridge. Back at the lab, Stradner diligently studies the recovering canine subject, but Mackenzie remains indifferent and unyielding regarding his plans. In a gripping turn of events, Navarro makes a brave bid to access the engine compartment, but his heroic attempt is thwarted by merciless guards, marking a crucial moment in the escalating tension.
Haley ultimately makes the ultimate sacrifice, selflessly giving her life to protect a young passenger amid chaos. Meanwhile, Kaplan, fueled by a desire for justice, uncovers a sinister plot to sabotage the train’s gas supply and critical coupler. Resolute, he ignites an explosion using his lighter, a courageous demonstration of his unwavering commitment.
After Kaplan's sacrifice, Chamberlain takes decisive action , successfully disconnecting the train’s rear segment to lighten the load and allow the remaining cars to cross safely. Tragically, fate intervenes as the hoped-for bridge collapses under the strain, resulting in the loss of everyone in the doomed front half.
As the surviving passengers are evacuated, they must face the stark reality of their newfound freedom — liberated from confinement and quarantine. In Geneva, a somber atmosphere envelops Stradner as he departs, weighed down by the haunting hope of potential survivors, while Mackenzie grapples with the guilt stemming from the catastrophic events that have transpired.
In a final twist, Major Stack discreetly relays to Mackenzie’s superiors that both he and Stradner are under vigilant surveillance, highlighting the relentless scrutiny awaiting them.
