This study aims to explore how a research-informed product, a hypothetical learning trajectory (HLT), was implemented by a group of Shanghai teachers in mathematics classrooms and promoted by a researcher through a Chinese lesson study. The instruction informed by the HLT served as a boundary object to promote conversations between the researcher and the teachers. The researcher played an indispensable role as a broker to facilitate the transformation of the boundary object. A fine-grained analysis of the data set showed that the team of teachers and the researcher formed a boundary encounter between teaching and research communities in which the participants negotiated and agreed on HLT-informed instruction. The study identified four turning points that are crucial for transforming HLT-informed instruction from an abstract object to a concrete form. Based on the turning points, we revealed the role played by the researcher as a broker in this lesson study. Finally, the challenges of bringing HLT into the classroom are discussed.