21st February 2015 at 10:25pm
When a tiddler is viewed or edited, then within its branch of the widget tree, the storyTiddler variable contains the title of that tiddler.
The default view template and edit template initialise storyTiddler to the value of the currentTiddler variable. This in turn will have been set by a $list widget in the relevant part of the page template.
storyTiddler is undefined outside the story river, such as in the sidebar.