Short answers, pulled from the story.
The Babington Plot was a 1586 conspiracy to assassinate Queen Elizabeth I and place her Catholic cousin, Mary, Queen of Scots, on the English throne. It was discovered by Elizabeth's spymaster, Sir Francis Walsingham, and used to entrap Mary in a plot against the queen.
The chief conspirators were Anthony Babington, a young Catholic gentleman, and John Ballard, a Jesuit priest who recruited him. Ballard hoped the plot would lead to the rescue of Mary, Queen of Scots.
Mary, Queen of Scots was executed by beheading on the 8th of February 1587, in front of 300 witnesses. She had been convicted of treason after a trial at Fotheringhay Castle in October 1586.
Walsingham had double agent Gilbert Gifford and cipher expert Thomas Phelippes placed inside Chartley Hall, where Mary was imprisoned. Letters were smuggled in a watertight box hidden inside a beer barrel, then intercepted, decoded, and copied by Phelippes before being resealed and sent on.
Mary had been imprisoned in England for nineteen years after fleeing Scottish rebels in 1568 and seeking aid from her cousin Elizabeth I. Elizabeth held her captive instead, under a series of jailers, chiefly the Earl of Shrewsbury.
Mary was held at Chartley Hall in Staffordshire, under the custody of Sir Amias Paulet, from 1585 onward. It was there that Walsingham's agents intercepted her secret correspondence with Anthony Babington.