It was plenty plausible in the comics. Michonne (pronounced "Mee-shawn", apparently - I'd been pronouncing it differently for ages now). is a major character in that media, and i hope it carries over here. I've believe that TWD has the same basic starting point in both comics and TV and have logically diverged since the end of the first episode. With Michonne and the Governor showing up next season I'd expect plenty other elements to be the same as well, but be believably handled differently in both cases.
This episode MOSTLY makes up for the preceding hours of people sitting around the farm talking about nothing (and not in the fun Seinfeld way). I'm not a fan of senseless zombie smashing, but TWD has a great story that sort of requires it for context. Unfortunately this season that great story was far too stretched out, and I think it will be remembered as such.
Without spoiling too much, the comics' prison arc has a LOT of story to mine for sixteen episodes (or more) next year, I hope that this (plus the budgetary savings of shooting on a studio or permanent set instead of on location all the time) will result in some great storytelling without sacrificing too much zombie mayhem. I hope that they advance the time frame too before Carl grows too far beyond where he started - his voice has already dropped noticeably, though it plays into his maturation as a young man.
I just hope they don't start next season with Carl being four inches taller without anyone noticing. :P