The process of plot composition in the context of interactive storytelling is considered from a fourfold perspective, that is, from the syntagmatic, paradigmatic, antithetic, and meronymic relations between the constituent events. These relations are shown to be associated with the four major tropes of semiotic research. A conceptual model and set of facilities for interactive plot composition and adaptation dealing with the four relations is described. To accommodate antithetic relations, corresponding to the irony trope, our plan-based approach leaves room for the unplanned. A simple storyboarding prototype tool has been implemented to conduct experiments.