Grey briefly meets Percy Wainwright in a London club for gay men, Lavender House, in June 1757. They meet again in January 1758, learning that they will become brothers by marriage, as Percy's step-father is to marry Benedicta Grey. Grey and Percy become close friends and lovers, and Percy joins Major Grey's regiment as a lieutenant.
In spring of 1758, they go with the army to Prussia, where Percy is caught having sex with a German soldier by Grey and two other witnesses. Percy is arrested for sodomy and awaits court-martial.
Hurt and angry with Percy's recklessness, but also worried for him, Grey visits Percy in prison. Percy tells Grey that he loved him, but his feelings weren't be reciprocated, as Grey is still in love with another man (Fraser), and asks Grey to save his life for the sake of kindness that was between them.
Grey is expected to testify against Percy, and knows that lying before the court-martial would destroy his reputation. However, he realizes that he cannot let Percy be hanged for a crime that Grey is also guilty of, but was simply never caught, and decides to help Percy escape from prison and flee the country.
Grey meets Percy again in July 1776 in Wilmington in North Carolina. Percy is now a French spy and uses the name Beauchamp, having married Cecile Beauchamp, one of the sisters of Baron Amandine. Percy wants Grey to convey an offer to the British government. He also mentions Jamie Fraser and Grey's stepson William in the conversation, and Grey asks him to stay away from him and his son – he clearly doesn't trust Percy.