A bumpy, but necessary, handoff between Influence Characters seals the structure of this time-travel narrative.
Time travel always wins, especially when you have Joseph Gordon-Levitt doing a delightfully dead-on version of a much-younger Bruce Willis (though at times the makeup comes off a bit sketchy in the direct sunlight). An extremely entertaining Sci-Fi yarn that unravels in often unique and surprising ways. Looper does, however, drag throughout the middle--an unfortunate occurrence explained by a shaky hand-off from one Influence Character to the other.
Two candidates arrive to fulfill this key element of a complete story: Old Joe and Sara. The former brings joy and ample opportunity for both acrots to shine, but with only one real scene to call their own (the one in the diner), the argument between young and old slips away into the fabrice of the space-time continuum (and the domain of the Objective Story Throughline).
Sara, the second candidate, arrives to fill in the gap left by Old Joe. While appearing out of left-field (or is it left cane field), their relationship offers solace for those who desire something more from their fiction.
And seals the time loop of narrative structure.
Relationship Story Throughlines support and administer growth of character, so its presence between Sara and Joe--while seemingly out of left-field (or is it left canefield?) is essential for the story to work.
Bumpy or not, Looper functions both as a complete story and an entertaining ride.