1939. As civilization teeters on the brink of war, ten strangers are persuaded to retreat from their troubled lives to Soldier Island, an isolated rock off the Devon coast.

As the sun rises and sets on the island and its solitary mansion, the number of guests is whittled down by their invisible enemy - to seven, to six, to five.

Cut down to half their original number, the guests of Soldier Island are in danger of losing their minds to fear and anxiety.