Seven Days to Noon (1950)

When Professor Willingdon becomes wary of the nuclear weapons he is helping build, he steals a warhead and threatens to detonate it in London in seven days unless the government begins nuclear disarmament. As Willingdon goes into hiding, Detective Folland of Scotland Yard sets out to find him. Willingdon's daughter Ann also joins the cause, hoping she can talk sense into her father before he causes a catastrophe.