Since 2016, Park School in Denmark has consistently used Japanese lesson study as an approach to the professional development of its mathematics teachers. The school has moved beyond the initial adaptation of lesson study, and no longer includes external support. In this article, I investigate how three groups of teachers participated in the maturing lesson-study phases, especially the reflection phase. I apply social practice theory, particularly the concepts of figured worlds and improvisation, to examine the groups' improvisation of artefacts and actions during lesson study, and the mature, valued outcomes. This study explains how and why the mature lesson-study efforts at Park School offer predominantly superficial learning opportunities to students and teachers alike. It concludes that in countries unaccustomed to lesson study, external support (or other external knowledge sources) is also important at the mature stages of its adaptation. This study proposes three activities that offer such support.