Time travel is impossible. But visiting and taking over a mind from the past, made receptive by madness --becoming a Voice inside the person's head, like a devil -- that's not only possible, it's Big Business. Only governments, corporations and universities can afford to send Voices into the past to gather eye-witness accounts of historic events.
To compensate for the immense mental strain and the ever-present threat of insanity, Voices are allowed to occasionally freelance. Usually they do it for money. But sometimes, for love. And that is the most and perilous motivation of all...