James Comey's firing as head of the FBI prompted shock, anger and chaos in Washington. In an excerpt from “Trump’s Showdown,” FRONTLINE’s season premiere, go inside Rod Rosenstein's role in that critical event—and see why the fallout almost prompted him to quit. “Trump’s Showdown” comes to PBS and pbs.org/frontline Tuesday, Oct. 2 at 9 p.m. EST/8 p.m. CST.