Gunpowder Treason and Plot

I've ignored Halloween this year in order to move straight on to Guy Fawkes night. I'm invoking a nursery rhyme here:

Remember remember
the fifth of November
Gunpowder treason and plot,
I see no reason why
Gunpowder treason
Should ever be forgot.

That reason is, of course, never explained. If the gunpowder plot is a new bit of British history to you here is that reason...