Tokyo Express by Seicho Matsumoto

'It was a puzzle with no solution. But he did not lose heart.'
 
In a rocky cove in the bay of Hakata, the bodies of a young and beautiful couple are discovered. Stood in the coast's wind and cold, the police see nothing to investigate: the flush of the couple's cheeks speaks clearly of cyanide, of a lovers' suicide.

But in the eyes of two men, Torigai Jutaro, a senior detective, and Kiichi Mihara, a young gun from Tokyo, something is not quite right. Together, they begin to pick at the knot of a unique and calculated crime...