The negative reviews here seem to be because this isn't just a history of Mary Queen of Scots: they're right, it's not, in that Mary is only born about halfway through this book. What it is, though, is an excellent history of Mary's inheritances, both personal and political, which shaped both her reign and her life.
Porter goes back to Katherine de Valois who, after the death of her first husband, Henry V, married Owen Tudor, a very unlikely second husband for a French queen. From here she traces the intertwined destinies of the Tudors and Stuarts whose lines merged when Margaret Tudor, sister to Henry VIII, married James IV, grandfather to Mary Queen of Scots.
So much has been written about the Tudors, but the lives of the difficult James III, the charismatic James IV, and the unlucky James V are far less well known, and Porter does an excellent job of bringing them to life. A minor criticism is that, given the English bias in current history writing, Porter feels it necessary to spend so much time on the reigns of Henry VII and Henry VIII.
So this does cover Mary's life, and does it in a fluent and lively style, but places her within the contexts of her family and national histories. For anyone wanting a more detailed focus on Mary Stuart, there's Antonia Fraser's established biography (Mary Queen Of Scots) and the newer, superlative book by John Guy (My Heart is My Own: The Life of Mary Queen of Scots).