When a tiddler is viewed or edited, then within its branch of the [[widget tree|Widgets]], the <<.def storyTiddler>> [[variable|Variables]] contains the title of that tiddler.
The default [[view template|$:/core/ui/ViewTemplate]] and [[edit template|$:/core/ui/EditTemplate]] initialise <<.var storyTiddler>> to the value of the <<.vlink currentTiddler>> variable. This in turn will have been set by a <<.wlink ListWidget>> widget in [[the relevant part of the page template|$:/core/ui/PageTemplate/story]].