Blood transfusion today is a routine medical procedure. Yet in the 17th century the idea of giving somebody blood rather than blood letting was dangerously unconventional. English pioneers like Robert Boyle, Christopher Wren and Richard Lower had performed a numerous experiments on animals, but nobody except Denis had dared to transfuse blood into a human being. Denis' idea was both simple and radical. By transfusing the blood of a calm, innocent calf into that of the dissolute and insane Mauroy he would cure him of his madness. The moral dilemma was obvious: Should he leave Mauroy in his misery and resign himself to obscurity or should he attempt to play God and earn his place in history?