Mary Queen of Scots was a very beautiful, tragic woman who was born to James V and Marie of Guise, she grew up as an only child and fatherless as he father died around the time of her birth, so she became queen. She grew up in France because she was betrothed to the Dauphin, Francis, son of Catharine de Medici and Henri III. When they were teenagers, she and the Dauphin married, but he was sickly and died as a teenager not long after.
So as a childless teenage widow she went home to Scotland to rule it. She was more French than Scottish in some ways because she had been brought up in France, not in fairly rough Scotland. So in some ways she never fitted in- her religion was Roman Catholicism, and that of many of the Scottish, Calvinism. She thus was some ways not suited to be Queen there. She had to find someone suitable to marry as she had to have heirs so she married Henry, Lord Darnely who was a cousin, descendent of Henry VII and of his daughter Margaret Tudor by her second marriage. She was infatutated with Henry, Lord Darnely but he was a foolish immature youth with little to recomend him beyond his royal blood and distant claim to the sucession to the English throne. Their marriage quickly went downhill once the infatuation wore off although they soon had a healthy son together, the future James VI of England. During her pregnancy with their son, he killed Mary's secretary an Italian Catholic named Rizzio because he thought Mary and this man were having an affair, although they were not. Henry and Mary's marriage became unbearable, but it ended when Henry died in mysterious circumstances among whispers he was murdered by Mary or men who were working for Mary. It was never proven, but whispers about Mary being a murderess soon sprung up to ruin her reputation.
She tried to rule Scotland after Darnley's death, but she was very unpopular and had a bad reputation, not helped by her marriage to the Earl of Bothwell, a Scottish noble who rumor had it had a hand in the murder of Darnley. So she looked bad married to a man who might have killed her husband or had a hand in it, it looked like she was to blame. But Bothwell had a lot of military power. She was soon expecting twins with Bothwell, but her marriage to him was very unpopular with Scotland. She eventually miscarried the twins and lost her throne in Scotland because even though Bothwell had military power, he and she both were very unpopular and she knew she was no longer welcome in Scotland, so after losing battles against forces in Scotland and being a prisoner for awhile in Scotland, she fled to England.
She always maintained as mentioned above she had a claim to the English throne since to her, as a Roman Catholic, the marriage of Elizabeth's parents had not been legitimate. She was a prisoner her whole time in England though because Elizabeth I felt she was a political threat, which she was. She plotted from her imprisonment in various plots to put herself on the throne and take Elizabeth off. She had been a famous beauty in her day but she grew older and her beauty faded in captivity, although early on in her captivity, part of one plot to put on her on the throne called for her to marry the Duke of Norfolk, a leading peer of England. Elizabeth was relunctant to exceute her but after she caught Mary in a plot directly she decided she had to execute her. So she was beheaded in 1587, a year before the Spanish Armada reached England. Elizabeth didn't want to, but she was compelled to have Mary put to death as Mary couldn't stop plottong in captivity her escape and Elizabeth's death. James, then a young man ruling Scotland ( in his childhood a regency had ruled for him) had never known his mother and thus didn't step in and also he wanted to have the English sucession, so that was another reason he did nothing. All in all, Mary's was a sad life. A good bio of her is the one by Antonia Fraser.
