loose end 
noun
A detail that is not yet settled or explained: Mark arrived back at his office to tie up any loose ends...
- The screenplay was so carefully written that everything is explained, loose ends are tied up.
- Those who like their loose ends tied should probably give it a miss, but if you're prepared to go with the flow, this is thrilling stuff.
- The plot does not follow a straight path whereby all the loose ends are tied up and everything finally starts to make sense.
Phrases
be at a loose end (or at loose ends)