"Partners both in throne and grave, here rest we two sisters Elizabeth and Mary, in the hope of one resurrection."

This inscription is visible on the tomb where Elizabeth I and her half sister, Mary I, lie buried together in one vault in the North Aisle of Henry VII's Lady Chapel in Westminster Abbey. It is the relationship between Elizabeth and her Scottish cousin Mary Stuart that is often discussed and pondered over while the relationship between Elizabeth and her own half sister is largely forgotten. Yet it is the relationship with Mary Tudor that forged Elizabeth's personality and set her on the path to queenship.

Mary's reign was the darkest period in Elizabeth's life. "I stood in danger of my life, my sister was so incensed against me," Elizabeth reminded her councillors when they pressed her to name a successor.It is time to tell the whole story of the fierce rivalry between the Tudor half sisters who became their father's successors.