Undergraduate courses in immunology are content-heavy and combined with a new, complex vocabulary, can be an overwhelming subject for students. In-class active learning approaches have been found to improve understanding of difficult concepts in science, technology, engineering and mathematics (STEM) disciplines; however, many undergraduate courses maintain a high dependence on lecture-style teaching because of time constraints, content demands and student resistance. We designed an online, out-of-class activity, the "Life and Death of a Tcell", to complement a lecture on a complex immunological concept, T-cell development. Inspired by the "Choose Your Own Adventure" children's books, a fictional narrative was created in which students assume the role of a cell with a dream of becoming a helper Tcell. Decision-making scenarios then prompt students to draw on their knowledge from the lecture to successfully navigate the steps of T-cell development. The activity was built on two platforms, Google Forms and H5P (HTML 5 Package), both of which are readily accessible and allow the inclusion of branching logic and the creation of a decision tree-based activity. An anonymous survey revealed that students found this interactive approach enjoyable, and their perceived understanding of the content significantly increased. Students appreciated the inclusion of a novel learning resource, with requests for similar activities to be developed for other immunological concepts. In conclusion, we developed a narrative-based, decision-making activity to complement a lecture on T-cell development. As an out-of-class activity, this style of learning approach can potentially capitalize on the benefits of active learning, while also overcoming barriers of student resistance.