In Sun and Steel, the author describes his childhood, particularly the development of his craft of writing as well as his exercise regimen of weight-lifting. Some of the larger themes include the idea of a "sound mind in a sound body," death, and the craft of writing.

As to the first, Mishima describes an intoxicating childhood experience watching a group of men carry a portable shrine through the streets. Mishima explains that, as a child, his abnormal conception of his body led him to be inattentive to it until seeing this group. The shrine porters looked up to the sky, and Mishima first wondered what vision they saw. Only later, once he discovered the language of the flesh, did Mishima learn that they were not seeing any vision, but simply looking up. Thus, the narrator explains that his early childhood privileged the cultivation of the mind over the body, when both were worthwhile pursuits. As an adult, Mishima spent considerable time lifting weights and noticed that by increasing the weight little by little, he could develop muscles of such unusual size as no modern man would require. This is similar to learning Classical Greek—a dead language whose revitalization required extreme discipline.

Mishima also admits his attraction to death. In the summer of 1945, he sees the sun blazing down on a field of barbed wire leftover from wartime. Mishima thenceforward associates the sun with death. Mishima admits that the attraction of an early death lay in its ability to be glorified but later realizes that an individual death cannot be glorious; rather, it must be a death enacted for a greater cause (such as the death of a warrior).

Finally, Mishima discusses the craft of writing. He likens works to corrosive stomach acid; just as the latter corrodes the stomach, words, of necessity distort reality. Mishima's childhood composition teacher never liked his writing, presumably (according to Mishima) because he was concerned with not representing reality via words; to represent reality would be (according to the young Mishima) failing to exploit words' power.