What was the Exeter Conspiracy?

On the 27th May 1541 the 67 year old Margaret Pole, Countess of Salisbury was escorted out onto the small green inside the walls of the Tower of London and beheaded in an execution which became infamous for how badly botched it was. This was the final dreadful act in a period known as the Exeter Conspiracy, which saw a total of 13 arrests and eight executions, with Margaret being the most notable victim, but what actually was the Exeter Conspiracy? Where did it start? Who was involved, and is there any truth to the accusations made?