The year is 1935. Clive Staples Lewis (known to his friends as "Jack") and his brother Warren (known as "Warnie") are visiting their friend young Tom Morris at Nesfield Cathedral School-where Jack is to be guest speaker at the school's speech night.
They become eyewitnesses to a murder-an impossible murder.
They see a man stabbed to death by an unseen assailant, and see his body fall from the rooftop... but the body doesn't hit the ground until the following day!
Once again Jack's nibble brain is co-opted by the police as they try to explain the seemingly impossible.