The Earls of Westmoreland and Northumberland, suspected of treason, go into rebellion, bringing in others such as Master Norton. They gather their forces, but are delayed in besieging a castle. Loyal forces defeat the rebels
The Percies of Northumberland and the Nevilles of Westmoreland were the great lords of the English north; on those rare occasions they agreed on anything, they could usually take Northumbria with them.
The north was also conservative; Catholicism was strongest there. By 1569, Elizabeth was securely Protestant, and her heir Mary Queen of Scots was in her custody.
The "Rising in the North" did not actually begin in Northumbria; Thomas Howard, the Duke of Norfolk, first contacted the Spanish about giving the throne to Mary. His interest, however, was political (he resented the power of the Cecils); he quickly backed down. (Though he would rebel again in 1571 and be executed.)
With the Duke of Norfolk out of the picture, the northern earls took over. They did not actually demand Elizabeth's overthrow -- but wanted Mary back on the Scottish throne and a restoration of Catholicism in England. The threat to replace Elizabeth with Mary was obvious.
The northern Earls succeeded in raising the north (including even Yorkshire, led by its sheriff Richard Norton), but they did not capture Queen Mary and could not bring the rest of the country to their banner (Holinshed says they gathered about seven thousand men, but even this may be exaggerated; such reports often are).
However large the rebellion was, it was dispersed by the end of December, with the rebels in full flight. Leonard Dacre tried to fan the flames in 1570, but he was easily suppressed. Some eight hundred rebels were executed.
For the sequel to this, see "Northumberland Betrayed by Douglas" [Child 176] and "The Earl of Westmoreland" [Child 177]. - RBW