The Sweet Mystery is one of Indiana's first paintings to include words. It features two doubled ginkgo forms, an element he began experimenting with in 1957, combining these earlier forms with the new visual language he was developing in the early 1960s. As in Terre Haute (1960) and the early construction Marine Works (1960–62), Indiana features diagonal “danger stripes," familiar from the construction sites, docks, and railyards of his lower Manhattan neighborhood.
The stenciled phrase "The Sweet Mystery" refers to the song "Ah! Sweet Mystery of Life," written by Victor Herbert and Rida Johnson Young, and performed by Nelson Eddy and Jeannette MacDonald in the 1935 Oscar winning film Naughty Marietta. Notable, in light of Indiana’s future work, is that “The Sweet Mystery” described in this sentimental song is “love”: "Ah! Sweet mystery of life / At last I've found thee / Ah! I know at last the secret of it all. . . . For 'tis love, and love alone, the world is seeking, / And 'tis love, and love alone, that can repay! / 'Tis the answer, 'tis the end and all of living." Contrasting danger stripes with the doubled ginkgo forms (Indiana’s symbol of “union”), the painting reflects a cautionary tale by Indiana of the dangers of love.