Historical Fiction, Tragedy
Lord Darlington, Stevens, and Miss Kenton are all fictional characters, but they interact with the big deal historical figures of the day. Winston Churchill, Herr Ribbentrop, and Sir Oswald Mosley—all real people—appear at Darlington Hall at one time or another. The novel is preoccupied with history and the way individuals shape (and are shaped by) it.
The storyline is also a textbook tragedy—no happy endings here. True to classical tragic form, we see a man born into power and tons o' cash (Lord Darlington) fall from grace due to his lack of knowledge and naiveté. Because of Lord D's hamartia, he ends up sinking his own ship.
Stevens's story/tragedy runs parallel to Lord Darlington's. He also falls from an ideal (for him) work situation—working in a "great" household, with colleagues he respects—into the relatively lonely station of butler to an American in an understaffed situation.