Short answers, pulled from the story.
The Babington Plot was a scheme to assassinate Queen Elizabeth I and replace her with Mary, Queen of Scots that occurred in the summer of 1586. The plot involved Catholic conspirators who planned to rescue Mary from her imprisonment at Chartley Hall and use her as a figurehead for a Spanish invasion of England.
The Babington Plot led to the execution of Mary, Queen of Scots because Sir Francis Walsingham intercepted letters hidden in a beer barrel that authorized the assassination of Elizabeth. Thomas Phelippes decoded these messages and forged a postscript to provide the legal justification needed to execute Mary on the 8th of February 1587.
Anthony Babington was a young Catholic conspirator who was recruited by Jesuit priest John Ballard to carry out the assassination of Queen Elizabeth I. He received coded letters from Mary, Queen of Scots, and the plot relied on his participation to rescue her from Chartley Hall.
Gilbert Gifford was a Catholic deacon who acted as a double agent for Sir Francis Walsingham to facilitate the movement of messages between Mary and her supporters. He secured access to the prison and arranged for letters to be hidden in a watertight box inside a beer barrel to bypass confinement orders.
Mary, Queen of Scots was tried at Fotheringay Castle in October 1586 by 46 English lords, bishops, and earls. She was executed by beheading in front of 300 witnesses on the 8th of February 1587.