What does Arby reveal to Nat in Chapter 19?

Arby reveals that it was his job to find someone to go back in time to save Shakespeare, and Nat had suffered strongly enough to do so. He tells Nat that Shakespeare was sad that he (Nat) never returned to the Chamberlain's Men after graduating from St. Paul's School. Arby also tells Nat that Shakespeare wrote a play inspired by his brief encounter with him in 1599. The play was called "The Tempest".