A Traveller in Time (1978)

Penelope lives in the twentieth century, and it is only when she goes to live with her aunt at Thackers, a remote, ancient farmhouse, that she finds herself travelling back in time to join the lives of the Babington family, and watching helplessly as tragic events bring danger to her friends and the downfall of their heroine, Mary, Queen of Scots, whom they are seeking to rescue.