Jeremy Paxman traces the story of the dramatic early stages of the war, from stunned disbelief to the mass recruitment of volunteer soldiers.
Fear of invasion grips the country, Boy Scouts guard bridges, and spies are suspected everywhere.
For the first time, British civilians are fired on by enemy ships and bombed from the air.
Paxman meets a 105-year-old eyewitness to the shelling of Hartlepool, who describes how she thought the Germans had landed.
Total war has come to Britain.
More about the series
This episode was first broadcast on Monday 27 January at 9pm on BBC One. For more information, and a link to watch again on the iPlayer where available, visit the BBC website.