When successful neurosurgeon John MacAndrew performs a routine operation to remove a tumour, the patient undergoes a severe personality change post-surgery. Hartman's Tumour is diagnosed, a rare condition which leaves its victims deranged and destined to be confined to mental institutions. There is no option but to have the patient committed. The patient's husband blames MacAndrew for the dreadful outcome and sets about to ruin his career. With an uncertain future ahead of him, MacAndrew retreats to his native Scotland to lick his wounds and it there that he makes further discoveries about the mysterious illness and the chemical that induced it. The damage wrought by the chemical affects the brain cells that normally block out a person's memory of past lives, with the result of the appearance of multiple personality disorder in sufferers. Armed with this knowledge, MacAndrew thinks he may be able to save his patient, until he discovers someone is deliberately using the chemical to regress selected individuals and gain eyewitness accounts of events in the past.