He's posing as a groomsman.
Rhys Sheffield, the Duke of Worthington, has bet his friends an ungodly sum of money that despite his loftiness, he can pass himself off as a servant at the house party of the Season.
But when his clever ex-flame arrives and recognizes Rhys in the stables pretending to be a groomsman, she realizes it's the perfect opportunity to pay him back for breaking off their engagement.
She's the bride to be.
The lovely Lady Julianna Montgome.