In 1932, the groundbreaking physicist Wolfgang Pauli met the famous psychoanalyst Carl Jung. Pauli was fascinated by the inner reaches of his own psyche and not afraid to dabble in the occult, while Jung looked to science for answers to the psychological questions that tormented him. Their rich friendship led them, in Jung’s words, into “the no-man’s land between physics and the psychology of the unconscious . . . the most fascinating yet the darkest hunting ground of our times.” Both were obsessed with the far-reaching significance of the number “137”—a primal number that seemed to hint at the origins of the universe itself. Their quest to solve its enigma led them on a lifelong journey into the ancient secrets of alchemy, the work of Johannes Kepler, and the Chinese Book of Changes. This is the captivating story of an extraordinary and fruitful collaboration between two of the greatest thinkers of the twentieth century.