As German occupation of Czechoslovakia in March 1939, Axel Bomasch (James Harcourt), a Czechoslovak scientist working on a new type of armour-plating, is flown to UK. Bomasch's daughter, Anna (Margaret Lockwood), is arrested before she can reach the airport and sent to a concentration camp, where she is interrogated by Nazi Germany who are after her father. Anna refuses to cooperate. Soon she is befriended by a fellow prisoner named Karl Marsen (Paul Henreid), who says he is a teacher imprisoned for his political views. Together they are able to escape and make their way to London. Anna does not know that Marsen is in fact a Gestapo agent assigned to gain her trust and locate her father.