Petri Nets can be used for a retrospective analysis of a computer game story, for representing plots in serious games as well as for monitoring the course of the story, as recently demonstrated by several authors. It was not clear, however, whether Petri Nets can be used for representing plots and for their unfolding in games that feature large worlds inhabited by virtual humans. This paper demonstrates that this is indeed possible, presenting several scenarios from the serious game Karo, an ongoing project, which features both virtual humans driven by reactive planning and a story manager unfolding story plots represented and controlled by Petri Nets. However, a specific refinement of Petri Nets, a hierarchical model capitalising on Timed Coloured Petri Nets and non-deterministic FSMs, had to be developed for this purpose. This refinement is described here, including general discussion showing why individual features of this refinement are needed - directions for game developers considering whether to use this technique, or not.