Mary fled to England in 1568 seeking her cousin Elizabeth I's protection, but her Catholic faith and her own genealogical claim to the English throne, which some English Catholics regarded as stronger than Elizabeth's own, made her far too dangerous a guest to release; Elizabeth kept her in a shifting series of English castles and manor houses under guard for nineteen years, a captivity that became the fixed point around which a succession of Catholic conspiracies against Elizabeth's life and rule organized themselves, each one aiming to replace the Protestant queen with her Catholic cousin.
The Ridolfi Plot, the Throckmorton Plot and finally the Babington Plot each drew Mary in as the intended beneficiary, and it was the Babington Plot, uncovered through intercepted and deciphered correspondence in which Mary appeared to approve Elizabeth's assassination, that gave Elizabeth's government the legal pretext to try her for treason and, in 1587, to execute her at Fotheringhay Castle. Mary's death did not end the association between her cause and Catholic restoration; if anything it sharpened it, and her son James's own accession to the English throne in 1603, uniting the two crowns, occurred only because Elizabeth died childless, leaving the Stuart line, Catholic in Mary's own person though Protestant in her son's upbringing, as the only viable succession left standing.