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Come on lads, let's get home, the sky's beginning to bruise, night must fall and we shall be forced to camp.
— Monty ‧ Withnail and I

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
Monty rouses his companions with mock-formal urgency, noticing the darkening evening sky and joking that if they linger any longer they'll be stuck sleeping outdoors. The line is playful and theatrical, treating an ordinary dusk as if it were an ominous turning point demanding immediate retreat.
Context reviewed