At the outbreak of war, two teenage boys - one German and one British - defy their parents to sign up. But their enthusiasm is tested when they get their first taste of battle.

1915. On the eastern front, Michael is forced to make a terrible choice, while on the western front Tommy's regiment experiments with a dangerous new weapon. When Tommy is immersed in battle – only to receive unexpected help while collecting the wounded – while Michael and his comrades engage in hand-to-hand combat after being ambushed by Russian soldiers on the Eastern Front. Returning home on leave, he is overjoyed to marry Katie, but struggles to reconcile his old life with the terrible things he has seen and done. Meanwhile, Tommy sustains a serious injury and recovers in an army hospital, where he takes a shine to x-ray assistant Joanna

1916. Tommy and Michael's regiments face one another on opposite sides of the Somme battlefield. The war reaches a stalemate, although both sides are fighting harder than ever. Despite this, the men’s hearts are elsewhere, and while Michael can only dream of his wife back home, the lads arrange for lovelorn Tommy to spend time at the hospital with Joanna. Back in the trenches, British and German troops gather at the Somme – and as the shelling begins, Michael’s hopes of getting out of there alive secretly dwindle.

1917. Michael is taken prisoner, and Joanna has some big news for Tommy. The soldiers are haunted by memories of their dead friends. However, Michael soon has more to worry about when his trench is attacked by British soldiers and he and young comrade Freddie are taken prisoner. Meanwhile, Tommy sinks toward despair, convinced the war is never going to end, so he talks his way on to the prisoner detail as a way of sneaking off to see Joanna at the hospital – and in their stolen time together, she has big news to tell him

1918. As peace terms are negotiated, Tommy and Michael are still on the front line. Tommy celebrates his son’s birth, Michael has to rely on letters from Katie to keep him going. Eventually, after four years of fighting, rumours fly through both camps of impending peace, but it’s a complicated process, and as politicians wrangle over terms and conditions, shelling continues on both sides. When a bomb takes out a barbed-wire fence, Tommy volunteers to mend it – bringing him face to face with none other than Michael. With the war almost over, do they fight or run?