After his little brother is born, like most normal children, the older brother/narrator is disturbed that the little boy is handicapped and "a burden" to the family. When the narrator observes that the baby named William Armstrong crawls backwards, he renames him "Doodle" like the reverse-walking bug, and feels that he has done this boy a credit since "nobody expects much from someone called Doodle." But, as he grows older, the brother views him as "a burden" because he must pull Doodle around in a cart and not let him get too hot or too cold, etc. When he has to "lug him everywhere" he goes, the brother begins to resent Doodle. But, one day he takes him to Old Woman Swamp where Doodle intuitively reacts to the beauty of Nature, and the brother becomes more sympathetic towards Doodle. Nevertheless, the narrator observes,
There is within me (and with sadness I have watched it in others) a knot of cruelty borne by the stream of love, much as our blood sometimes bears the seed of our destruction, and at times I was mean to Doodle.
This strange quality in the hearts of many is what propels the narrator to push Doodle to learn to walk, swim, row a boat and other activities requisite of boyhood so that he will not be ashamed of him. But, this "knot of cruelty" that takes the brother's urgings beyond the realistic capabilities of Doodle become the nemesis of the afflicted child and the great burden of the brother's heart, who has caused his "scarlet ibis" to strain beyond his capacities and die.