Based on findings from the research project ‘Reaching Young Audiences’ on film, serial fiction and storyworlds for children and young audiences (RYA 2021), this paper analyses the writing of the television Christmas calendar Christmas of the Comets which was shown as 24 episodes on the commercial public service broadcaster TV 2 in December 2021. The Christmas calendar genre is quite unique to Scandinavian television as a very popular form of fiction that gathers families for shared viewing during the month of December (Agger 2013). The TV 2 television Christmas calendar of 2021 was unique in the way that the idea for the series came from the Niels Bohr Institute at the University of Copenhagen where astrophysicist Anja C. Andersen and television producer Marie Breyen developed a plan to create a greater interest in the natural sciences through the making of an inspiring fictional universe marked by fascinating natural science elements. TV 2 and the production company Nordisk Film were fond of the idea and hired showrunner Jenny Lund Madsen to head the writing of the series. Based on literature on writing for children and family audiences (Brown 2017; Hermansson and Zepernick 2018; Redvall and Christensen 2021) and qualitative interviews with Jenny Lund Madsen, Marie Breyen and Anja C. Andersen, document analysis (of e.g. the press material, reviews) and observation studies at industry events (such as THIS Series festival in 2021), the case study explores the process of creating a Christmas calendar with an arena and a storyline where natural science elements are naturally integrated in each episode. Building on theories of transmedia storytelling (Ryan 2015), the case study also explores the extensive transmedia universe ‘Universet udenom’ (the universe around) which was created around the series and the way in which this mirrored the action and conflicts of the fictional story.