Sterling is a 4-year old Deerhound Scottish dog that belongs to Mary Stuart. He was brought over from Scotland to France for his mistress.
Stirling is most likely named after Stirling Castle where Mary was crowned Queen and raised when she was a little girl.
Sterling hears noises in the woods, and after barking at it, he runs off. Mary tries to follow, but Sebastian stops her. He promised to find her dog for her and to bring him back.
Later that day, Bash finds Sterling and returns him to Mary, who greets him with a big hug, and kisses.
Mary Stuart was sitting in her room by th fire place petting Stirling when King Francis walked in. They had a short conversation about their relationship, all the while Mary kept petting Stirling's ears.
Sterling was in Mary's Chambers and was waiting for her by the fire place.
Later that night, he wakes up when he hears Mary panic. He sit at her side for comfort as she talks with her husband.