I am trying to figure it out. Even more now that Robert Plant has done his own version. It sounds so wonderfully dark my imagination takes me away. I imagine a psycho boyfriend rushing out of the city to a dark, dark, country road to kill his girlfriend. Or, he is running a light he knows they can't make to kill them both. She has obviously let him down for the last time. That's what I'm hearing. The "monkey" is the bad feeling he has been carrying around for way too long and he needs to get it off his back, once and for all. Plant's version is even a little more eerie. We can only wonder.