It's from "Angels Passing" by Graham Hurley, a crime novel set in Portsmouth. It's a very long conversation between cops: "If Finch's off the plot, he'd be all over her." To give context I'd have to type in a couple of pages! Finch is in a flat with a good looking black girl, "pissed out of his head", while other two men(bruisers) who want to shag the girl, call her twice on the phone and want to teach Finch a lesson(beat the shit out of him). Hope this helps.
Sounds to me as though off the plot is just a variation of lost the plot.
Which in itself is a metaphor, an allusion to a writer of stories or plays who has got confused and lost the thread of his story.