Most of the action takes place before Lincoln's election to the presidency. As I read Grahame-Smith's account, I had to keep checking Wikipedia to read the real biographies of Lincoln, Mary Todd Lincoln, John Seward, and a dozen others--which I always enjoy. The plot revolves around Lincoln's drive for revenge against vampires for murdering his mother. As he slashes his way across the midwest with his trusty ax, Lincoln discovers that vampires are exploiting slavery in a bid to eventually take over the entire country.
That's pretty much it for plot, actually. There are a lot of great fight scenes and plenty of dramatic tension (apart from knowing that Lincoln himself can't die until 1865). It wasn't a bad way to pass the time.