While never a big statistical performer for the Invaders, in his rookie 1983 season Plummer nevertheless played a big role in helping the Invaders capture the Pacific Division title, intercepting three passes and recovering a fumble.  He would add two more picks in 1984 and one in 1985.  After the 1984 season, Invaders coach John Ralston boldly stated that Plummer was better than any linebacker on the Bay Area's other team - the NFL champion San Francisco 49ers.

Staying with the USFL to its end, Plummer moved on to the NFL's San Diego Chargers, where he played for eight years and was a fan favorite.  In 1994 he returned to the Bay Area, joining the San Francisco 49'ers, where he helped the team win Super Bowl XXIX - against the Chargers.  Plummer would play three more seasons before retiring from pro football in 1997 at the age of 37.

Recently Plummer has become outspoken about the use of steroids in pro football, testifying before Congress about his experiences.  While Plummer claims to have never used steroids himself, he testified that he had seen its use first-hand while with the Chargers,  "I remember walking in and seeing two linemen inject each other... it was a shock.  I remember hearing about it but I had never seen it."  Plummer summarized NFL training camps in the late 80's as "like the WWF."  He apparently at least considered taking steroids himself, but after seeking a doctor's advice on the subject opted not to.  Today Plummer remains connected both to the game and to the fight against steroid abuse, serving as a color commentator with the 49'ers radio network and training young athletes, counseling them on the potential dangers of steroids.