Plot's mysterious quality (8)
I believe the answer is:
intrigue
This clue is a double definition.
'plot's' is the first definition.
(intrigue is a kind of plot)
'mysterious quality' is the second definition.
(I've seen this before)
(Other definitions for intrigue that I've seen before include "An underhand plot or conspiracy" , "Secret plotting" , "liaison" , "Plot, conspire" , "Illicit plotting" .)