She had been a queen of Scotland. Many Catholics considered Queen Elizabeth's reign to be illegitimate and that Mary was the true and rightful heir to the throne.
She was on trial for her part in the Babington Plot to assassinate Queen Elizabeth.
The trial only lasted two days. The evidence against her was damning.
But it would be weeks and months before she was finally punished, and was executed.
The trial and execution was in Fotheringhay Castle, which was destroyed by Mary's grandson Charles I in 1627.
Shakespeare would have been 22 at the time, and in London already or well on his way to the city.
The trial and subsequent execution of Mary would have been a very frightening time for people in England, whether you were Catholic or Protestant.
There had been violence against Catholics and Protestants before, but to have the Queen execute her (second) cousin was something entirely different.
Whatever feelings people had of her, by August of 1588 and the victory over the Spanish Armada, there was no doubt in anyone's mind that Elizabeth was all-powerful.
In my versions of Hamlet and Richard III I show what it must have been like for Shakespeare to arrive in a London that feared and loved their Queen in equal measure.