To have students in university teacher education courses independently explore what a “good lesson” is, there is a need to first eliminate the gap between university courses and courses students have taken up through high school and disentangle the formalities unconsciously shaped during their K-12 experiences. Keeping this in mind, this study focuses on students who, after enrolling in university classes, have accepted a theory of teaching different from that which they had supported up until then. These students were able to create high-quality lesson plans, and this study investigates and analyzes why they were able to heighten their lesson planning skills. Based on analyses of the lesson plans of three students and interviews with them, the following three points were identified: (1) students tried to reflect upon and refine their own lesson plans by being enlightened by others’ purpose-rational lesson plans, (2) their own views of social studies and theories of teaching were relativized by learning about multiple theories of teaching with different goals, and (3) they had little formality with regard to the subjects and fields (geography) that they had not taken during high school. Rather, they were able to independently envision educational content in a purpose-rational fashion. Furthermore, our findings suggest the potential of guidance for supporting the improvement of social studies lesson planning skills that (1) draws from within students criteria and conflicts with regard to the their own latently-held ideas of a “good lesson” through the experience of evaluating exceptional classes taught by others and creating their own lessons (rather than forcing upon them good lessons from the outside through university classes), and (2) makes them understand the importance of engaging in teaching materials study and content construction in accordance with subject objectives, using as material subjects and fields that students have not fully understood the importance of.