In 2005 Plummer experienced arguably his best season as a professional. He threw 229 passes without an interception, the longest such streak of his career. He also began serving as Denver's holder, which he would continue to do through the 2006 season, as well. Along the way, Plummer helped the Broncos compile a 13–3 record, a #2 seed, and a first round bye in the AFC playoffs (behind the Colts). The Broncos' first game was against the New England Patriots in the AFC Divisional Game at Invesco Field. Although not outstanding, Plummer's performance (15–26 for 197 yards, 1 touchdown, 1 interception) helped the Broncos break the Patriots' winning streak of 11 postseason games and gain their first postseason victory since Super Bowl XXXIII. However, Plummer accounted for four turnovers in the AFC Championship game and the Broncos were defeated 34–17 by the eventual Super Bowl champion Pittsburgh Steelers.