The plan is Walter's idea of what they should do when they make the hand off with the kidnappers, the Dude just wants to do whatever the kidnappers say. The dude gets a phone call from the kidnappers with instruction to throw the bag out of the moving vehicle when they cross a wooden bridge. This, according to Walter, fucks up their plan. Walter tells the Dude to grab the ringer, which is a bag filled with his dirty whites in lieu of the briefcase with the alleged cash. They throw the dirty undies over the bridge, Walter rolls out of the car asking The Dude to grab the wheel and keep the car at 15 MPH and then calls for the Uzi (an Israeli-made automatic gun wrapped in a mailing envelope and duct tape). Walter rolls out and loses control of the gun which shoots a few rounds. The Gran Torino continues with Walter losing control of the car and he crashes into a phone poll. The Dude grabs the real briefcase in an attempt to salvage the hand off and runs down the road trying to catch up to the kidnappers. The kidnappers flee the scene in motorcross bikes.The interesting part is that Walter apparently just wanted to "come along".
"It's a swiss fuckin watch, If I understand it correctly."
"This is it, Dude, let's take that hill!"