In this epistle she intimated her wish that Lord Buchan would devise a plan to surprise Bothwell Castle the ensuing day, to prevent the departure of its armed vassals, then preparing to march to the support of the outlaw Sir William Wallace, who, with his band of robbers, was lurking about the caverns of the Cartlane Craigs.
When this letter arrived, Lord Soulis was at dinner with the other lords; and Buchan, laying it before De Valence, they all consulted what was best to be done.

Lady Mar begged her cousin not to appear in the affair himself, that she might escape the suspicions of her lord; who, she strongly declared, was not arming his vassals from any disloyal disposition toward the king of England, but solely at the instigations of Wallace, to whom he romantically considered himself bound by the ties of gratitude.

As she gave this information, she hoped that no attainder would fall upon her husband.

And to keep the transaction as close as possible, she proposed that the Lord Soulis, who she understood was then at Dumbarton, should take the command of two or three thousand troops, and marching to Bothwell next morning, seize the few hundred armed Scots who were there ready to proceed to the mountains.

She ended by saying that her daughter-in-law was in the castle, which she hoped would be an inducement to Soulis to insure the Earl of Mar's safety for the sake of her hand as his reward.
The greatest part of Lady Mar's injunctions could not be attended to, as Lord de Valence, as well as Soulis, was made privy to the secret.
The English nobleman declared that he should not do his duty to his king if he did not head the force that went to quell so dangerous a conspiracy; and Soulis, eager to go at any rate, joyfully accepted the honor of being his companion.