This qualitative exploratory study aimed to examine how preservice teachers constructed unit plans with the assistance of ChatGPT. Due to the constantly evolving landscape of technology and rapid development of Generative AI (GenAI), teacher education programs are compelled to prepare preservice teachers with the necessary skills to integrate these advanced technologies into their daily teaching practices. Grounded in the conceptual framework of technology, pedagogy, and content knowledge (TPACK), this study delivered an in-depth analysis of the processes through which preservice teachers interacted with ChatGPT when creating unit plans. It further explored factors that preservice teachers considered when using ChatGPT to support curriculum design. Findings from the study illustrated preservice teachers' complex and nuanced interactions with ChatGPT. It underscored the importance of leveraging technological knowledge, content knowledge, pedagogical knowledge, and contextual knowledge for preservice teachers in the digital age. Findings also suggested that the current GenAI, such as ChatGPT, can support preservice teachers in completing certain teaching tasks, such as unit plan design, but is not capable of replacing the teaching profession. Implications drawn from the study may provide insights for teacher education programs on how to effectively prepare future educators to utilize AI technologies.