Mary impetuously married her cousin, Lord Darnley, against the wishes of Scottish nobles. Her affection for him soon failed. He was murdered in 1567.
From a public domain image
Mary, Queen of Scots, with her second husband, Lord Darnley
Whether Mary was involved in the murder of Darnley has been a controversy ever since the murder happened. Bothwell -- Mary's next husband -- has often been blamed, and sometimes Mary herself.