Tristan was the second son of Graeme Muir and lived with his family on Gordon lands in the north east of Scotland. After the defeat of the Scottish forces were his beloved father and brother were killed helping the newly crowned Robert the Bruce make his escape, Tristan became disillusioned with the Scottish cause and headed south to England were he felt that he could in some way seek out the perpetrators of his kin's deaths.  In time Tristan became friends with Henry Fenes who was close to the English court, Henry like Tristan had a  grudge with the English, however Henry suspected himself of being watched to closely by the English but still needed to pass on valuable information to the Scots regarding English intensions to his contact Sir Aidan de Culter. Tristan agreed to be the go-between for Henry after a while came to respect Sir Aidan who reminded him of his father.

Tristan was introduced to the Scottish lords Sir Andreas of Thurso and Sir Harry de Brechin by Sir Aidan who was passing on information to them from Henry Fenes. Tristan was not aware that Sir Harry had dealt with Henry directly put due to pressing matters elsewhere he had asked Sir Aidan to take up this role.  Now Sir Harry had learned of Henry's capture and requested men to help find and rescue Henry. Sir Aidan suggested Tristan and Sir Harry provided help in the form of David de Brownhill who had experience dealing in such matters. Tristan disliked de Brownhill from the start for David treated Tristan with contempt at every opportunity; however Tristan knew he could not let his personal feelings get in the way, he had an important mission to accomplish. Eventually the two men found out the location of Henry and was able to gain entry to were he was being held. Just when they were to gain access to Henry's cell Tristan found himself alone, de Brownhill had vanished. Time was against him and he continued to break into Henry's cell. Once Tristan had explained to Henry what had happened Henry's expression hardened for Henry knew of de Brownhill and the kind of man he was.

Before Tristan knew what had happened Henry had picked up Tristan's shield seconds before an arrow embedded itself into the shield. Tristan ran after the man who had fired the shot only to find it was de Brownhill. In the struggled that followed De Brownhill was wounded before making his escape.  Tristan knew that neither Henry nor he were safe and so the two men travelled to Scotland where Tristan took Henry to a safe house were Henry could recuperate from his imprisonment more speedily. Tristan returned to court in order to tell  them of de Brownhill's treachery only to be told that he was the one to be imprisoned for treason - de Brownhill had arrived in Scotland first and had influenced everyone to believe that Tristan was the traitor. Everyone except Sir Aidan who could not believe the man he had become so attached to would stoop to such a thing. Sir Aidan requested the help of his mentor Michael Beckett to use the power of the church to help uncover who really was behind all of this. De Brownhill was captured and tortured, from his confession it was learned that de Brownhill had been paid by Paul de Brechin, Sir Harry's half brother to kill Tristan and put the blame ultimately at the hands of Sir Harry, in this way Sir Harry could be discredited and Paul could claim the lands of Brechin for himself and his paymaster the Earl of Buchan.

Tristan was released and now worked directly with Sir Harry taking on the role from Henry Fenes as Sir Harry's contact at the English court. After Bannockburn, Tristan was able to use his influence to establish talks between the two courts which ultimately led to the Treaty of Edinburgh.

In later years Tristan worked closely with John Mackay 2nd laird of Thurso in orchestrating the release of King David II from English hands after the battle of Neville's Cross.