A Dangerous Liaison tells the intense, passionate and sometimes painful story of how these two brilliant free-thinkers - and rivals - came to a share a relationship that was to last over fifty years.

Moving from the corridors of the Sorbonne and the chestnut groves in the Limousin, to the cafes of Paris's Left Bank, we discover how the strikingly beautiful and gifted young Simone came to fall in love with the squinting, arrogant, hard-drinking Jean-Paul. Seymour-Jones describes that first summer of 1929: the heated debates that went on long into the night, the sexual rivalry and betrayal, the dangerous ideas that led people to experiment with new ways of behaving and the deep love that this perhaps unlikely couple shared.

We hear how Sartre clandestinely compromised with the Nazis and fell into a Soviet honey-trap. And, thanks to recently discovered letters written by de Beauvoir, the darker, more dangerous side to their philosophy of free love is revealed, including Simone's lesbianism and her pimping for younger girls for Jean-Paul, in order to keep his love.

This is a compelling and fascinating account of what lay behind the legend that this brilliant, tempestuous couple had created.