Once a Soldier takes place in Washington, DC, and recounts the story of LTC Marco Domenico, a 39-year-old Delta Force operative. During his extraction from an unnamed country teetering on the brink of insurrection, the helicopter in which he is a passenger crashes. Marco suffers severe injuries - ones that could prevent his ever walking again.

His sister, Guiliana Leoni, is unable to leave Italy in time to wait for his arrival at Walter Reed. She calls her former college roommate, Ay'ana Levy, and wakes her from a dream in which she is exploring the Ethiopian highlands with a handsome man. When Ay'ana realizes the seriousness of Guiliana's call, she agrees to wait for Marco's arrival at the medical center despite her belief their mutual antipathy won't provide much support. Marco and Ay'ana have never gotten along, and after Marco's successful surgery and subsequent recovery, their animosity toward one another escalates. Guiliana is helpless to stem the deteriorating situation.

As the story progresses, Marco faces obstacles in his efforts to learn to walk again, ones that also challenge his view of himself as a man. He realizes no matter how mobile he becomes, his army career has changed, and he must accept those changes and make plans for the future. His progress in his efforts to overcome his physical injuries and regain his confidence as a warrior are remarkable, but the situation with Ay'ana Levy continues to degenerate to the level of internecine warfare. At the urging of his physical therapist, Marco makes an effort to meet Ay'ana halfway, and the two begin to build a better relationship - one that holds the potential for a brighter future for them both.