Finland has recently reformed the national school curriculum, basing it on so-called twenty-first century skills. The curriculum prescribes that schools must organise at least one multidisciplinary learning module per year. In this case study, we investigate the implementation of such a module in secondary school (grade 9) from the perspective of the school headmaster and science and mathematics teachers. The data comprise interviews with the headmaster, teachers, and protocols from collegiate meetings. Despite some gains experienced by teachers concerning cooperation with colleagues, the results reveal challenges connected to diffusely defined learning goals, as well as to the vagueness of goals concerning the role of different subjects in the module. The study shows the importance of offering research-based support and time for shared sensemaking for stakeholders to avoid the problems identified when implementing multidisciplinary teaching.