The divide between theory and practice is an enduring challenge commonly attributed to disconnection in teacher knowledge between 'knowing what to do' (planning) and 'doing what you know' (enacting in class). We re-visited this divide from the new perspective of pedagogical content knowledge with the purpose of examining the ways in which topic-specific pedagogical content knowledge (TSPCK), evident in a planning-to-teach setting, translates in a classroom setting. Furthermore, we aimed to identify the factors that influence the translation. We employed an intervention-based qualitative research design with a subset of three science pre-service teachers (PSTs) drawn from a cohort of 24 final year PSTs in a Chemistry methodology class. They were followed into schools for observation during a practicum. Data collected comprised lesson plans, two video-recorded lessons each and self-reflection reports. The lesson plans and the lesson recordings were mined for TSPCK episodes, while the reflection reports were analysed for possible influential factors. The identified TSPCK episodes from both the lesson plans and actual lessons were subjected to a qualitative, in-depth analysis, and the results were collated into TSPCK maps. The reflective reports were analysed through content analysis. Findings revealed a structural format change of the PSTs' TSPCK across the planning-practice settings, while keeping the original lesson intentions intact. Factors influencing the observed TSPCK structural change included complex teacher reflections-for-action and reflections-on-action drawing on specific TSPCK components as well as on learner understanding. Implications for science teacher education are discussed.