Frank Tannen was a Sergeant in the United States Army and was stationed in Hill Valley in 1944. Marty McFly and Verne Brown encountered Sgt. Tannen after traveling to the past to find the lost blueprints for a machine that Doc had invented.
Frank looked like, acted like, and sounded like, Biff Tannen. It seemed likely that Frank was Biff's father, as well as the son of Gertrude Tannen (who, by 1955, was "the only Tannen in the book"). However, this is not the case.
Sometime after 1944, Frank no longer lived in Hill Valley, and by 1955, Biff lived with his Grandmother Tannen. It is unclear whether Sgt. Tannen was a casualty of World War II or the Korean War, or whether he had been transferred away from Hill Valley during his Army career, or had simply moved away after a tint in the Army.
Behind the scenes
- In the episode, Frank uses German expressions twice, calling Marty a "weisenheimer" and a "dummkopf", and corrects himself both times. However, his conduct as a sergeant in the United States Army does not appear to show allegiance to any other country.