Plaster graduated from high school in 1967. He was trained as a communications sergeant in the Special Forces. He served three combat tours in Vietnam as a member of SOG, leading intelligence-gathering and recon teams in North Vietnamese Army-controlled areas of Laos and Cambodia and along the Ho Chi Minh Trail. He was wounded once, and decorated four times, eventually receiving a field commission in recognition of his combat experience. He retired from the military at the rank of Major.