A Communist officer falls hard for a married woman trying to escape from Hungary.

In 1956 Hungary, a national uprising against Communist rule results in Soviet military occupation. At the Budapest airport, several foreigners anxiously waiting to depart are informed that flights have been canceled, but they will be transported to Vienna by bus. British television representative Hugh Deverill is surprised to find acquaintance Lady Diana Ashmore among the waiting passengers. Diana hopes to keep secret that she is traveling with Hungarian national Paul Kedes, who is hiding a severe wound from a street battle and is traveling under an assumed British identity. Although weak and exhausted, Paul insists that Diana disassociate herself from him in the event that he is detained. As the bus departs Budapest, Diana startles Hugh by confiding that she is divorcing her respected British husband. After the bus passes a Soviet roadblock, Paul faints and Diana rushes to his aid, raising Hugh's suspicions. Further down the road, the bus is halted by a group of Hungarian freedom fighters led by the steely Eva. Allowed to proceed, the bus soon arrives in Mosen, the last Hungarian town before the Austrian border. The officer in charge of the border check point, Major Surov, confiscates the passengers' passports and informs them that they must be individually questioned before being allowed to cross the border. Upon preliminary questioning, Surov is struck by Diana's poise and beauty, and also notes Paul's veiled hostility. At the local hotel, proprietor Csepege welcomes the travelers and, at the major's orders, places the men in quarters apart from the women. Sharing a room with Simon Avron and Teklel Hafouli, Paul collapses into bed with a high fever. Hugh warns Diana that if Paul is wanted by the authorities, Diana's association with him may place all the bus passengers in jeopardy. When the others go downstairs to dine, Diana slips into Paul's room to rebind his wound and Paul begs her not to place herself in danger. Surov attends the group's dinner and notices Diana's late entrance and Paul's absence. When gunfire outside interrupts the meal, Surov wonders why the Hungarians hate the Soviets. After dinner, Surov's impervious demeanor is shaken by the death of young freedom fighter killed just outside the hotel. That night, when Paul falls into a delirium and speaks an unfamiliar language, Simon awakens Hugh to express his and Teklel's fear that Paul is not British. Roused by the murmur of the men's voices, Diana overhears their discussion, then follows Hugh downstairs. Over tea, Diana reveals that Paul is a Hungarian biologist, whom she met and fell in love with in England years earlier. After Diana ended their relationship and married someone else, Paul returned to Hungary where his correspondence to England regarding her prompted government suspicion that he was a spy. Arrested and tortured mercilessly, Paul spent five years in prison and was just released. Still in love with Paul and wracked with guilt for his suffering, Diana is determined to get him out of the country. The following morning at breakfast, Csepege announces new regulations about reporting all Hungarian nationals and surrendering arms. Led by Simon, the men demand that Diana turn Paul over to Surov, but she refuses. When Surov arrives with forms for the passengers to fill out, he notices Paul's continued absence, but no one in the group gives him away. Diana brazenly offers to fill out Paul's form, then later takes it upstairs to him to sign. Surov follows and in a moment alone with Paul, accidentally finds and confiscates his gun. Later when Paul and Diana realize the gun is missing, Paul declares they must leave immediately. Diana asks Csepege how Paul might be smuggled across the border and he directs her to a contact at the fish market. After making the arrangements to escape that night, Diana is picked up by the military police and Surov. Taking Diana to a deserted brewery, Surov shows her Paul's gun and demands to know why Diana has been lying. Diana is startled when Surov admits that although he can arrest Paul, Diana's presence has forced him to hesitate and doubt his previous unquestioning attitude. That evening at dinner, Surov assures the anxious passengers that their trip will likely resume the following day and demands that the occasion be festive. Csepege secretly advises Diana that the escape plan is confirmed, but Diana is detained when Surov drunkenly insists that they dance together. Diana finally flees the dining room, but on her way to join Paul, Surov stops her to apologize for his behavior and to return Paul's gun as a farewell gift. Moments later, Diana joins Paul and a guide in a small boat in the canal. As the boat floats by the guard tower, Surov arrives on horseback and Diana and Paul are caught. Paul struggles to defend Diana, insisting that she is not responsible. Back at the hotel, the remaining passengers are placed under house arrest and several wonder why Diana and Paul did not inform them of their escape. Back at his office, Surov files his report on Paul's arrest, ignoring Diana's emotional pleas to spare Paul. Later, Surov visits Paul in jail and is affected by his courage and humanity despite his bitter experiences. Meanwhile at the hotel, the passengers, led by pregnant American Margie Rhinelander, accuse Diana of instigating their difficulties. Margie declares that everyone has noticed Surov's attraction to Diana and bluntly encourages Diana to take advantage of it to help save them. While riding back to the hotel from the jail, Surov's horse is shot out from under him by Eva hiding in a tree. Distressed, Surov returns to the military headquarters, but is unable to kill his suffering horse. When Diana arrives, Surov bitterly chastises her naïve idealism, then admits that her influence has ruined him. When Diana confesses that she came at the behest of the others, Surov is outraged, then demands that Diana confess that she returns his feelings. Despite sharing a passionate kiss with Surov, Diana departs. Early the next morning, the group, minus Paul, returns to the bus. A few miles away, as the relieved passengers disembark at a small bridge leading to Austria, Surov drives up in a jeep with Paul. Diana rushes to thank Surov, but he assures her that he has acted only to clear his conscience. Moments after Diana and Paul are welcomed to Austria by the border guards, shots ring out as Surov is killed by Eva.