The poem, of some four hundred and sixty three lines, is written in five-line stanzas with a varying rhyme scheme. It was first published in the Lyrical Ballads of 1798, where it appeared between The Mad Mother and Lines Written Near Richmond.
The poem is narrative in form. Set in the countryside, it tells the story of Betty Foy and her mentally handicapped son. Foy's neighbor Susan is sick; Foy has no choice but to send her son into the nearby village to fetch the doctor. She places him on her pony and sends him on his way. When he has not returned after several hours, she grows worried and sets off to find him. The doctor has not seen the boy; finally, she finds him placidly astride his pony, who is grazing near a stream. As they are walking home, they encounter Susan, who has, as it were, worried herself well and come in search of her friend.
In the Preface to the 1800 Lyrical Ballads, Wordsworth remarks that the purpose of this poem, as of The Mad Mother, is to trace "the maternal passion through many of its more subtle windings." It is, then, one of the volume's poems concerned with mothers, and Betty Foy, in her love and care, may be considered a natural foil to the bad mothers of The Mad Mother and The Thorn. The poem is also a clear demonstration of the principles Wordsworth laid out in his prefaces; it is a poem illustrating common emotions in a rural setting, using plain language and eschewing the formality of most eighteenth-century verse.
Wordsworth is concerned mainly with psychological experimentations, using speakers who are very much like himself. These speakers do not connect with the characters portrayed in the poems beyond the simple stereotypes that the narrators provide as characterization. The impotence of the poet-narrator in the presentation of the poem prevents him from harnessing the creative power that Wordsworth identified as a "spontaneous overflow" of joy within a man of genius: the poet. In "The Idiot Boy," both the mother Betty and her idiot son, Johnny, achieve the spontaneous overflow that characterizes a successful creation, but the poet cannot, and repeatedly laments his inability to describe in poetic form what the story contains. This poem, like many in Lyrical Ballads also concerns itself with the psychological insights of the mother of the idiot boy showing her clear concern for her child.