She wasn't allowed any visitors and letters were checked. Her jailer was a strict Puritan.
The idea was to push Mary into another plot.
She began a secret correspondence with the French ambassador, Babington.
By mid 1586 a plot to kill Elizabeth and free Mary had been designed.
Unknown to Mary, Elizabeth's "spymaster" knew of the plot and the letters.
On 17th July 1586, Mary wrote a coded letter approving of the plot.
In October Mary was placed on trial for treason.
In December 1586, Cecil prepared a death warrant, but Elizabeth refused to sign it.
Only in February of the following year did Elizabeth sign the warrant.
On 8th February 1587, Mary was beheaded.