A group of English Catholics led by Robert Catesby planned to blow up Parliament during the State Opening and kill the Protestant king. They had moved 36 barrels of gunpowder into a cellar beneath the House of Lords.
An anonymous letter warned a Catholic lord to stay away, and a search around midnight found Fawkes with the powder. The other plotters were caught or killed within weeks. The British still mark the night with bonfires and fireworks.
Source: Wikipedia