The future of the world is changed when America joins the conflict and the Communists come to power in Russia.
Ian finally delivers his message to James Stirling but in order to secure Susan's release he and Barbara are forced into a dangerous spying mission.
The French Army is rallied for a great spring offensive – but mutiny looms when the plans fall foul of the Germans.
Both sides make heroic bids for victory – but what will they have to show for it?
The Doctor is forced into a meeting with Robespierre while Barbara's attempts to find a physician for Susan result in them being betrayed.
Susan and Barbara are rescued by Jules and Jean, two members of the escape chain, while the Doctor gains access to the prison by posing as a revolution official.
The Battle of the Somme – which began in July 1916 and continued into February 19 – leaves 415,000 Brits in a muddy grave.
Following Lord Kitchener's appeal in 1916, the British Army was reinforced by over two million volunteers. Despite the reservations of General Sir Douglas Haig, the new Commander-in-Chief, many of these mew recruits were sent to the scene of Britain's biggest military endevour – The Battle of the Somme.
Susan, Ian and Barbara are sentenced to be guillotined but an encounter with a dying English prisoner may offer Ian a chance of survival.
The TARDIS lands during the French Revolution, where Ian, Barbara, and Susan become caught up in the politics of the day. When they are arrested and set to be executed, the Doctor must take on the disguise of a public official to try and save their lives.
One of the bloodiest battles in history, the fighting at Verdun continues throughout the year 1916. French and German forces suffer over 700,000 casualties.
1915 brings little progress for the Allies, but a huge German offensive at Gorlice-Tarnow forces the Russians to retreat in the East. The Allied conference at Chantilly agrees on a strategy of simultaneous attacks upon the Central Powers in 1916.
While the City Administrator's schemes reach their climax, the Doctor and Ian venture into the tunnels under the city to discover the real cause of the Sensorite plague.
The City Administrator is determined to destroy the Doctor and his friends and his paranoia prevents the Doctor's cure from being distributed.
By Christmas 1914, the bloody impasse in the trench lines on the Western Front saw Lord Kitchener looking elsewhere for victory. In an attempt to sieze control of strategic Turkey, a British naval expedition advanced on the Dardanelles, and allied troops landed at Gallipoli with the aim of taking Constantinople. However, events did not go according to plan and, after 37 weeks, soldiers were evacuated from the beaches.
As Minister of Munitions, David Loyd George was faced with a restricted industrial capability; Britain even imported materials from Germany in key areas. But with the help of US equipment, Lloyd George brought troops up to date, and by 1916 had transformed the armed services into a modern military power.
With Ian having fallen victim to the Sensorites' plague, the Doctor is desperate to find a cure. But the City Administrator is still determined to rid his world of aliens.
The Doctor and his companions are escorted to the Sense-Sphere where they attempt to negotiate with the Sensorites for the release of the TARDIS.
In 1915, France and Britain struggle to keep up with the demand for shells and artillery, but Germany – the most modern industrial system in Europe – have no shortages, having smoothly switched over to war production and introduced the use of gas made by chemical factories. The Allies discover democracy and laissez-faire industry will not suffice if they are to regain ground.