Guide one through the story line (5)
I believe the answer is:
pilot
'guide' is the definition.
(piloting is a kind of guiding)
'one through the story line' is the wordplay.
'one' becomes 'i' (Roman numeral).
'through' means one lot of letters goes inside another (I've seen this in other clues).
'the story line' becomes 'plot' (I've seen this before).
'i' put within 'plot' is 'PILOT'.
(Other definitions for pilot that I've seen before include "Plane operator" , "Ship's guide" , "Person in charge of aircraft" , "-- Whale; -- light" , "Plane driver" .)