Final answer:
Mary, Queen of Scots, was involved in a conspiracy to overthrow Elizabeth I and unite England and Scotland under her rule. She was a strict Catholic and faced execution for treason in 1587.
Step-by-step explanation:
The individual involved in a conspiracy to overthrow Elizabeth I with English and Spanish Catholics was Mary, Queen of Scots.
Mary, a strict Catholic and first cousin to Elizabeth I, sought to take the English crown and unite England and Scotland under her rule.
Her actions led to her being accused of treason, found guilty, and she was subsequently executed in 1587, which was a tumultuous time marked by religious and political strife, including the rise of the Puritans and the impending threat of the Spanish Armada.
Mary's plot to overthrow Elizabeth I and the ensuing intrigue are central to understanding the complex religious dynamics of the period as well as the fragility of Elizabeth's own reign.