Elizabeth I (person)

Edward died in 1553 from illness and named Lady Jane Grey as his successor. He had been tricked into this by Lord Dudley who planned to rule the throne through his son's wife, Jane. The poor girl ruled for only nine days before being overthrown by the rightful heir, Mary Tudor. Once Mary was on the throne, Elizabeth was the center of Protestant plots to get rid of the Catholic Queen, who was unpopular and nicknamed "Bloody Mary" for her persecution of non-Catholics. Mary did imprison Elizabeth for a while, but Elizabeth convinced Mary that all the plots were going on without Elizabeth's knowledge or involvement, and she also attended Mass with Mary, which pleased Mary just as Elizabeth's refusal to officially convert pleased English Protestants. Mary also believed she was pregnant at the time and felt more secure on her throne with the supposed coming of an heir to it (though she turned out not to be pregnant; it's now thought that it was some kind of cancerous uterine tumor). She died in 1558 and Elizabeth became queen but the throne wasn't secure yet.

Mary, Queen of Scots was Elizabeth's first cousin once removed (her grandmother was Henry VIII's sister) who felt she had a better claim to the throne of England than Elizabeth did, because Mary was Catholic and because Elizabeth had been called illegitimate after her mother's death. Mary spent 20 years plotting to gain the English throne, much of it from prison. When a letter was discovered where Mary agreed to Elizabeth's murder by Mary's supporters, Elizabeth had no choice but to have Mary executed in 1587. Oddly enough, however, when Elizabeth died (of tonsillitis, according to the only source* that names a disease) unmarried and childless in 1603, Mary's son James I, who had ruled Scotland for 36 years, also became King of England.

*(David Williamson's The Kings and Queens of England)