I'm comparing a list of Achievements from the interet to the Achievements file on my computer, and I can't quite figure out what this one is for. I got it on my first playthrough when I had the Twins, the Time Traveller, and the Monk.
It's listed right after 'God Bless Us Everyone' and right before 'Every Hour on the Hour' (which is probably 'Prevenge'), so it was earned sometime between the Twins and the Time Traveller's quest.
The Twins were my first characters. Maybe there's an unmarked Achievement for completing the first plot?