During World War II, Kaplan composed music for the film units of the U.S. Army Signal Corps and the Office of Strategic Services, the predecessor to the CIA.
His film career was disrupted during the 1950s when he landed on the Hollywood Blacklist after being uncooperative in testimony before the House Committee on Un-American Activities.
Jeff Bond noted: "Although he wrote only two scores for the series, New York composer Sol Kaplan's music was tracked frequently throughout the show's first two seasons".
He was subpoenaed by House Committee on Un-American Activities (HUAC) after John Garfield mentioned during his testimony that Kaplan was a friend of his.
Kaplan protested that many top studio executives were friends of Garfield, including the man firing him, and he was reinstated on a week-to-week "probation" basis.