When a core element of the plot is a mystery. The most common variation would be a detective murder mystery (where the question is most often "Whodunnit?"), but also very popular in Noughties Drama Series', Jigsaw Puzzle Plots and Survival Horror. In many cases the question is equivalent to, "What the hell is going on and why?" which taken far enough makes for an Ontological Mystery.
See The [...] [tvtropes.org]