The title tells us where we are: the morning after, or the muddle of a night of drinking. The opening line makes it plain. A "richly appointed headache" is a funny, almost showy way to describe pain, as if the hangover were a fancy hotel room the speaker never booked. That mock-grand phrasing suggests a speaker who can joke about his own misery.
Then the poem slips into a dream, and the logic goes sideways. A gun shoots water instead of bullets, so the threat is harmless, yet the speaker still runs. He swings from a branch onto a moving truck, like an action-movie scene shrunk down to something silly. The last line lands with a small sting: he is caught "empty handed," a phrase that hints at guilt, failure, or simply having nothing to show for the chase.
The mood is wry, woozy, and a little anxious. The poem feels like the speaker's foggy head: funny on the surface, with unease underneath.