The plot is simple. Mrs Patmore is outside having a crafty smoke before putting the veg on when she notices the Reverend Albert Travis stumbling around the courtyard looking, for all the world, as if he is well drunk. Worried that his Lordship might see him she quickly hustles the vicar into the kitchen only to find out, as he starts feasting on her ample flesh that he has been zombified. After finishing off the cook, Reverend Travis goes on to infect all of the servants and together they climb the stairs to get their revenge on the b**tards who have been making their lives hell for the last six series.
Robert Crawley's final words (in fact, they are the final words of the final episode of the final series) are, "Come on, chaps. This really isn't British."
Then they rip his head off.