Francis II had one of the shortest reigns in French history. Born in 1544 to Francis I and Catherine de' Medici, his grandparents belonged to the houses of Valois-Angoulême, Valois-Orléans, de' Medici, and La Tour d'Auvergne. He was betrothed to Mary, Queen of Scots as a child, and married her in 1558. He was styled king of Scotland after this marriage, and later in the same year, king of England and Ireland, because Catholic France didn't accept Elizabeth as the rightful monarch. Finally, in 1559, he became king of France after the death of his father. Unfortunately, he died after a reign of only one year and 59 days, due to meningitis, although poisoning was widely suspected at the time.
Upon his death, the throne passed to his brother, Charles IX, who was only 10 years old. Catherine de' Medici exercised power through a regency. Mary, now Queen Dowager of France, left for Scotland despite the protests of her friends.
In the What If? type of historical speculation, if Francis II had lived long enough to produce a male heir, it's possible that one prince would have had four crowns: France, England, Scotland and Ireland.