The present study investigates the use of generative artificial intelligence (AI) tools by pre-service teachers (PSTs) in lesson planning for a middle-school English as a foreign language (EFL) class, aiming to address gaps and inform teacher training. The case study examined PSTs in a South Korean university course who were tasked with creating lesson plans using generative AI to aid in lesson plan development for a middle school lesson that incorporated generative AI. Data were analyzed thematically, and results revealed that generative AI was used in topic selection, material creation, lesson organization, and language checking. While generative AI facilitated efficiency and creativity, challenges emerged, including the quality of outputs and limited incorporation of effective pedagogical strategies. These findings indicate a need for targeted training in prompt engineering, ethical considerations, pedagogy, and collaborative practices to enhance PSTs' generative AI competencies. This study contributes to teacher education programs by providing insights into the practical integration of generative AI in pedagogical practices.