The guy in the blue jersey wearing No. 50 in the video above is Chuck Plzak. (It's a primitive recording, taken in 1984, so try to ignore the sounds of heavy breathing.)
Plzak was a middle linebacker for the Dallas Wildcats, a semipro team playing the Fort Worth Drillers. As you can see, at various points Plzak blocks a punt for a touchdown, intercepts a pass, makes a ton of tackles, and strips a ballcarrier before taking the ball back for another touchdown—all in the same game.
The video was sent to us by Plzak's nephew, Tyler. Plzak played in college at Wisconsin-Stevens Point and also played a season of semipro ball in Rockford, Ill. He said he had tryouts with the Dallas Cowboys and Miami Dolphins, plus the USFL's Houston Gamblers, San Antonio Gunslingers, and Los Angeles Express.
This is Plzak's take on the above video:
I was 24 years old. I had games similar; I had just tried out for the Cowboys in '84 but didn't make it and got asked to play in this semipro league. When they voted for the all-league team, your coach was supposed to nominate you. But my coach told me before he could even try, three other coaches stood up and nominated me. I had 25 tackles in one game. We were undefeated but lost in the championship game. It just makes me wish I would have tried harder to get another shot in the NFL.