He fought in Vietnam with the 1st Battalion, 7th Cavalry Regiment. He participated in the Battle of Ia Drang in Vietnam in 1965, under the command of Lieutenant Colonel Hal Moore, who praised Plumley as an outstanding NCO and leader in the 1992 book about this battle, "We Were Soldiers Once… and Young". The book was the basis for the 2002 film "We Were Soldiers", in which Plumley was played by actor Sam Elliott. Plumley was known affectionately by his soldiers as "Old Iron Jaw".