Elizabeth Tudor, or Elizabeth I, acceded her father, Henry VIII, to become the Queen of England at 25 years of age. Being a Protestant, she knew she had a tenuous hold on her power. Her cousin, Mary Stuart, or Mary, Queen of Scots, a Catholic, was seen by many English Catholics as the rightful heir to the throne. That is, until she married her third husband, who had been accused of murdering her second husband. Taking advantage of the moment, Elizabeth denounced Mary as an adulteress and imprisoned her.
For 19 years, Mary, along with a skeleton crew of servants, was moved by Elizabeth between various small castles and manors throughout England. Mary's supporters concocted a plot to overthrow Elizabeth and install Mary as monarch in Elizabeth's place. Through coded messages, Mary arranged the details of the plot with her supporters.
Without their knowing, however, Elizabeth was in fact the person who triggered the assassination plot on her own life. A new Act had been passed by Parliament which allowed the execution of any person who would benefit from the murder of the Queen. Elizabeth shrewdly played her pieces and Mary eventually got the axe.
Interestingly, though the feelings between the two cousins over the course of their lives were...intense...to say the least, Elizabeth and Mary never actually met face-to-face.