After one year at Shasta College, he was signed by the St. Louis Cardinals on April 25, 1965, as an amateur free agent, and was named a Florida Rookie League All-Star that summer.

While playing for Sparky Anderson with the Modesto Reds in 1967, Phillies scout Eddie Bockman noted that Plummer was "strong, can catch everyday," has "all the desire and hustle in the world," and "recommend a Rule 5 draft on him if he is available.

Plummer made his major league debut with the Cubs on April 19, 1968, at the age of 21 in a 9–2 road loss to the Cardinals.

[5] The Cubs traded Plummer, Clarence Jones, and Kenneth Myette to the Reds for Ted Abernathy on January 9, 1969.

He was in the minors again in 1970, but was called up to the pennant-winning Reds in September, long enough to play in four games with nine plate appearances, including his first career hit.

[7] While never a regular starter—he was Johnny Bench's backup catcher during the Big Red Machine years—he did play solid defense with a .983 fielding percentage, but was a lifetime .188 hitter.

His most memorable game was in 1974, when he hit two home runs in Philadelphia off Hall of Famer Steve Carlton.

He doesn't drink, works out, jogs and plays tennis, and during the winter he labors on his father-in-law's northern California cattle ranch.

"[9] After he retired as a player, Plummer stayed in the Mariners' system, and managed the San Jose Missions in 1980, the Wausau Timbers in 1981, the Chattanooga Lookouts in 1984 and 1985, and the Triple-A Calgary Cannons from 1986 through 1988.

When third-year manager Jim Lefebvre was fired after the 1991 season, the franchise's first with a winning record,[16] Plummer was promoted for 1992.

In 2002, he joined the Arizona Diamondbacks' minor-league system, eventually working his way up to their Triple-A affiliate, the Tucson Sidewinders,[35] which he managed in 2007–2008.