Plummer was drafted in the 2nd round of the 1997 NFL draft by the Arizona Cardinals. He played behind Kent Graham and Stoney Case at the start of his rookie season. He took his first snap late in the 4th quarter of game 7, and promptly led the Cardinals on a 98-yard drive, going 4-of-6 for 87 yards and capping it with a 31-yard go-ahead touchdown. He led the Cardinals to 3 of their 4 victories that year. Already locally popular from his days at ASU, according to teammate Chad Carpenter he was now treated "like a god. We go to a restaurant and people stand up and clap when he walks by." In 1998, the Cardinals drafted Plummer's friend Pat Tillman, and the two started all 16 games "en route" to a 9–7 regular season record. In game 10 against Dallas Cowboys, he threw for a stellar 465 yards and three touchdowns. In the playoffs, he led the Cardinals to an upset of the same Cowboys for the franchise's first postseason victory since 1947, before losing in the second round to the Minnesota Vikings.