As stated in the mission, it's a reference to the phrase "It was a dark and stormy night; the rain fell in torrents," which was part of the introductory sentence of an 1830 novel called Paul Clifford by Edward Bulwer-Lytton.
The full sentence is "It was a dark and stormy night; the rain fell in torrents, except at occasional intervals, when it was checked by a violent gust of wind which swept up the streets (for it is in London that our scene lies), rattling along the housetops, and fiercely agitating the scanty flame of the lamps that struggled against the darkness."
Did I ilulz right?