As the threat of the Nazi regime looms large over Europe, translator Harry Chase vows to help his Polish lover Kasia flee Warsaw, but how will he explain this to his sweetheart Lois, waiting for him at home in Manchester? Meanwhile, American journalist and broadcaster Nancy Campbell is at the Polish-German border, making a grisly discovery. In Paris, surgeon Webster O'Connor finds the distant threat of war on his doorstep as he falls for French-African jazz musician Albert Fallou, who has been beaten up by the Action Francaise mob.

In devastated Warsaw, Kasia makes plans to flee with grief-stricken mother Maria but gets caught in a resistance cell's attack on German soldiers. Harry returns to Manchester. When he finally plucks up the courage to tell Lois the truth about Warsaw, she disarms him by declaring her love. Harry then travels to London where he's dismissed from the diplomatic services because of his conduct in Warsaw. Tom shocks Douglas by announcing he is now a conscientious objector, while in Berlin Nancy is frustrated by the growing power of the censor. In Paris, Webster and Albert are deeply in love, but their joy is overshadowed by the growing threat of prejudice they face.

After enlisting in the navy, Tom finds himself on board the HMS Exeter in pursuit of the notorious Graf Spee, a German pocket battleship which has been picking off British merchant ships for months. A huge naval battle ensues, and Tom's moral compass is reset as he watches the carnage and death around him. As Tom fights for his life, Harry and Lois face a battle of their own. Harry is now a 2nd lieutenant, and is in France with the BEF, supervising the digging of foxholes and tank traps with his sergeant Stan, and trying to win the respect of his unit. When Lois arrives at the base camp to perform with ENSA, Harry's two worlds collide. Lois is determined to ignore him and finds comfort in the arms of another soldier, Joe, who happens to belong to Harry's unit, and there's another shock in store for Harry when he later disciplines Joe...

The phoney war is over. Harry and his unit are fighting for their lives in the town of Louvain, where German troops far outnumber those of the Allied forces. Harry is desperate to prove himself as a leader, but when the Nazis close in, Harry freezes. Can he regain the respect of his men? On the road to refuge Grzegorz and Konrad are woken by a German soldier. Should they fight or escape? Meanwhile, back in Warsaw, Kasia and Tomas's resistance activity has horrific consequences as the Germans swiftly retaliate. In Paris, Webster is shocked to learn more of the rise of anti-Semitism in the city, when a Jewish couple come seeking his help. Despite hospital policy, Henriette persuades him too look after them, revealing that she too is Jewish. Secrets are also uncovered in Manchester, when Douglas guesses that Lois is hiding her pregnancy. Tom's return brings momentary relief - until he confesses that he plans to desert the Navy.

As thousands flee Nazi-occupied Belgium, Harry, Stan and the rest of their unit take the last route out of France - the road to Dunkirk. When the boys finally make it to a field hospital, they find an overwhelmed Webster O'Connor dealing with an endless number of injured soldiers and civilians. Having stumbled across an ambulance of shell-shocked, traumatised troops (including Senegalese soldiers Demba and Ibrahim), Harry is faced with a dilemma - should he stay with the men or save himself? In Warsaw, Kasia's campaign of violence and murder against German soldiers has become routine; she's worlds away from the young, carefree woman she used to be. Her part in the emerging Polish Resistance is firmly established. In Manchester, Lois and Connie are performing with Ensa at a northern airbase. They meet Flight Lieutenant Vernon Hunter who is immediately drawn to Lois, charmed by her candour and humour.

June 1940. The Nazis have taken Paris with not a bullet fired. Webster and Albert's life in the city they love will never be the same again. In Berlin, Nancy finds herself once again at the centre of the Rosslers' increasingly desperate situation as both Mr and Mrs Rossler are arrested for neglecting their patriotic duties. Back in Manchester, Harry returns home with another companion, Demba, one of the heroic Sengalese troops who stayed by Harry's side at Dunkirk. And in Warsaw, Kasia's murderous routine continues in earnest. As she and Tomasz take ever-greater risks in targeting the Germans, the net closes in on them.