The purpose of this study was to, first, examine the potential of lesson plans (LPs) of preservice science teachers (PSTs, N = 631) as a source for gathering rich data on their pedagogical content knowledge (PCK) and, second, qualitatively describe PSTs' reasons for the level of data richness in their LPs on PCK. PSTs' handwritten LPs were analyzed through an analytic rubric, and semi-structured interviews were conducted with 79 randomly selected PSTs. Results revealed a range of the level of data richness of the PSTs' LPs on PCK and five descriptive categories about the PSTs' reasons for the richness of LP data on PCK. The study implies that researchers should consider the inclusion of the LP as a valid tool in their triangulation design; however, they should weigh the factors that influence the collection of rich data in their PCK studies to triangulate findings better to portray a holistic picture of PCK.