[4] Old Man's War is the story of retiree John Perry and his exploits as a soldier in the Colonial Defense Forces (CDF).

The characters in Old Man's War have enhanced DNA and nanotechnology, giving them advantages in strength, speed, endurance, and situational awareness.

John Perry, a 75-year-old retired advertising writer, joins the Colonial Defense Forces who protect human interplanetary colonists.

Volunteers sign letters of intent and provide DNA samples at age 65, which John and his now deceased wife Kathy had done ten years prior to the beginning of the story.

Following a series of sometimes unusual psychological and physical tests, Perry's mind is transplanted into a clone of himself made from collected genetic material.

He now possesses enormous strength and dexterity, nanobot-enhanced artificial blood, enhanced eyesight and other senses, and most critically, a BrainPal—a neural interface that, among other capabilities, allows Perry to communicate with other members of the CDF via thought.

After a week of frivolity and libertinism in their new bodies, Perry and the other recruits land on Beta Pyxis III for basic training.

During this training, the CDF's heritage in the United States armed forces is made clear when the recruits are taught the Rifleman's Creed.

By the end of this last engagement Perry begins to suffer psychological distress over killing the Liliputian Covandu and accepts that he has transformed both physically and mentally.

Perry's quick thinking allows him and his fellow soldiers on a transport shuttle to escape the wreckage of the Modesto and make for the planet's surface, but they are shot down.

Perry thinks he has died when he sees a younger green version of his dead wife Kathy, who in reality is Jane Sagan, the leader of the Ghost Brigades rescue team.

Sagan and Perry then participate in a Special Forces operation in an attempt to capture or destroy the borrowed Consu technology in advance of a major attack to recapture Coral from the Rraey.

Perry is instrumental in the successful outcome of the battle by capturing the technical manual for the Consu detection system (which was destroyed in the fighting), and saving Sagan's life after she is severely wounded.

At the conclusion of the book, Perry is promoted to captain following his deeds at Coral and, despite the separation, holds hope of reuniting with Sagan when their terms of service conclude.

The BrainPal is a neural implant that allows members of the Colonial Defense Forces (CDF) to send and receive data, including speech, battle plans and much more.

To the members of the Ghost Brigades the BrainPal does much more: it provides a synthetic consciousness that allows the newborn soldiers to function until their own identities develop.

When presented with a situation that is unfamiliar to the newborn soldier, the BrainPal loads the relevant and important information directly into the mind at an amazing rate.

They became a serious problem for the CDF after acquiring technology from the Consu to predict the trajectory of a vessel's skip drives, a feat that was previously considered impossible.

[12] Old Man's War sits in the military science fiction genre but themes of the ethics of life extension, friendship, marriage, the significance of mortality, what makes one human, and individual identity are present within the novel.

[citation needed] The themes of marriage and friendship are explored in the characterization of John Perry through his continued love of his dead wife and his later meetings with Jane.

[17] Recent developments suggest Steven Spielberg will be signing on to direct a feature version of the book in 2026, following the release of his own original sci-fi film 'The Dish.'

A character can be seen reading the book in an episode of the science fiction television series Stargate Universe,[18] as a shout-out to Scalzi in his role as creative consultant on the show.