Complex sentence: U.S. Army Col. Robert Neville, M.D. (Charlton Heston), a scientist based in Los Angeles, begins to succumb to the ensuing plague but manages to inject himself with an experimental vaccine just in time, rendering himself immune.
Split into simpler sentences: U.S. Army Col. Robert Neville, M.D. is a scientist based in Los Angeles. As he begins to succumb to the plague himself, he injects himself with an experimental vaccine, rendering himself immune.