Robert Catesby didn't like the king. He knew that was bad, as the king had this 'divine right' thing, but how could he help that? The late queen had been alright, letting them worship in peace, but the new king actively prosecuted Catholics. Robert himself was a Catholic. Not that he was going to admit to it in a busy tavern, of course. He'd be in court before he could say Catholic. He called over a nearby person. He whispered "Are you Catholic?" "Yes", came the response. "Want to help me do away with the king?". "Alright". The gunpowder plot had begun.
At first, they tried to dig a tunnel under Parliament to blow it up, but this failed. Instead, they rented a cellar underneath and put some gunpowder in. However, the king, fearing an attack, changed the date he would be in Parliament at to the 5th of November. In the meantime, the gunpowder expired... so the Catholics had to buy more.
But everything was about to get worse... One of the plotters was related to an MP, and so sent him an anonymous note, warning him not to go. The MP gave it to the chief of spies, who gave it to the king. Guy Fawkes was captured and tortured for information on the others.