Given the task definition and input, reply with output. In this task, you're expected to write answers to questions involving multiple references to the same entity. The answer to the question should be unambiguous and a phrase in the paragraph. Most questions can have only one correct answer.

Passage: The film opens with Sir John Falstaff and Justice Shallow walking through the snow, then to a warm fire inside the Boar's Head Tavern, as the two reminisce. After a main credit sequence, the narrator explains that King Henry IV of England has succeeded Richard II, whom he has killed. Richard II's true heir, Edmund Mortimer, is a prisoner in Wales, and Mortimer's cousins Northumberland, Worcester, and Northumberland's son Hotspur demand that Henry rescue Mortimer. The king refuses, and thus Mortimer's cousins begin to plot Henry's overthrow.
To Henry's great dissatisfaction, his son Prince Hal spends most of his time at the Boar's Head Tavern, drinking and carousing with prostitutes, thieves and other criminals under John Falstaff's patriarchal influence. Falstaff insists that he and Hal should think of themselves as gentlemen, but Hal warns Falstaff that he will one day reject both this lifestyle and Falstaff. The next morning Hal, Falstaff, Bardolph, Peto, and Poins disguise themselves in Gadshill to prepare to rob a group of traveling pilgrims. After Falstaff, Bardolph, and Peto rob the pilgrims, Hal and Poins jump out in disguises and take the stolen treasure from Falstaff as a joke.
Back at the Boar's Head Tavern, Falstaff begins to tell Hal and Poins with increasing exaggeration the story of how the money was stolen from him. Hal and Poins poke holes in Falstaff's tale until they reveal their joke to the entire group. In celebration of the newly recovered stolen treasure, Falstaff and Hal take turns impersonating Henry, with a cooking pot crown and vocal impressions. Falstaff's Henry chastises Hal for spending his time with common criminals, but names Sir John Falstaff as his one virtuous friend. Hal's Henry calls Falstaff a "misleader of youth".
Question: Who begins to plot to overthrow a king?
Northumberland.