Fiona is the good buddy and scientific confrere of the young William Thomson, newly appointed to the chair of Natural Philosophy at Glasgow University in 1849. Unconventional, uninhibited, and brilliant, she provides the keys to his scientific discoveries. She also takes it upon herself to tutor him for his forthcoming marriage bed with his young, priggish, cousin. Fiona herself has to marry quickly as a cover for her and William's lovechild, and settles into a life of frustratingly respectable motherhood in Belfast, while William's career in science and engineering takes off into a knighthood and eventually a peerage. Fiona and William manage occasional passionate trysts for 20 years, but finally, after a crisis, have to give each other up. Rooting out the letters that document the secret Fiona/William affair involves adventure, passion, and another outrageous Fiona.