Story line involved arsenic and love (8)
I believe the answer is:
scenario
'story line' is the definition.
(I've seen this in another clue)
'involved arsenic and love' is the wordplay.
'involved' is an anagram indicator (involved can mean confusing or complex).
'and' says to put letters next to each other.
'love' becomes 'o' (love means zero in tennis).
'arsenic' with letters rearranged gives 'scenari'.
'scenari'+'o'='SCENARIO'
(Other definitions for scenario that I've seen before include "Dramatic situation" , "Outlines of drama" , "Master plan" , "Event postulation" , "Storyline" .)