A Bad Dream of Death — John Newton Chance

Revenge has many shapes. In the game of espionage the shape is usually death. But there is another kind of death - that of the mind. When a man wakes to find he cannot tell nightmare from reality, he is on the road to Limbo, at the frontier of Hell. Jonathan Blake realised he was on that road when he could no longer tell whether the nurse bending over him with the needle was a sister of mercy or his murderer...