After spending a decade in Arizona, Plummer signed as a free agent with the Denver Broncos in 2003, replacing Brian Griese. Though the laid-back Plummer would clash often with the domineering head coach Mike Shanahan, his guidance helped Plummer finish with a career-high 91.2 quarterback rating. On September 22, he had the longest run by a Broncos QB on Monday Night Football, a 40-yard scramble in a 31–10 win over the Oakland Raiders. He led the Broncos to a wild card playoff berth, where the Broncos were beaten 41–10 by the Indianapolis Colts.