In order to prevent Cid (Pierce Gagnon, in an incredible performance) from growing up to become "The Rainmaker", Young Joe (Joseph Gordon-Levitt) shoots himself in the heart with his own Blunderbuss as Old Joe (Bruce Willis) is about to shoot Cid's mother, Sara (Emily Blunt). This causes Old Joe to immediately disappear as if he never existed at all. We are left to presume that since Cid now grows up in a loving environment and Sara finds the stash of silver bars that Old Joe stole when he wiped out Abe (Jeff Daniels) and his gang, that Cid's destiny has been changed.
Joe is a "Looper". A Looper is a hired assassin that kills people who have been sent back from the future. In the future, time travel is illegal, but criminal organizations use this technology to eliminate their adversaries.
When a Looper's future self is sent back, the Looper in the present must kill his future self. He gets a big payday, and it's his last kill, but he lives with the knowledge that his days are numbered.
When Joe's older self is sent back, Young Joe fails to kill him. This leads Young Joe to be hunted by his criminal boss in the present, Abe.
Old Joe and Young Joe meet in a restaurant and Old Joe reveals that he intends to kill "The Rainmaker", a devastating individual of the future who uses his telekinetic powers to wreak havoc and destruction singlehandedly. It is "The Rainmaker" that is sending back all of the future Loopers to be eliminated. When Old Joe was sent back, The Rainmaker's men killed Old Joe's wife, who rescued Old Joe from a life of debauchery.
Old Joe has some information about the birthday of The Rainmaker and plans to kill the Rainmaker as a child. The problem is there are three potential candidate children.
When the restaurant is surrounded by Abe's men, both Joes escape and head in separate directions. In the melee, Young Joe tears the map that Old Joe has marking the location of the three children to be killed. The map leads Young Joe to the farm of Sara, who has a young child, Cid. Old Joe seeks out the other two children.
Old Joe kills one of the kids, while Young Joe learns that Cid has telekinetic powers that explode uncontrollably when Cid is angry. This culminates in the spectacular death of bounty hunter Jesse (Garrett Dillahunt). Cid was abandoned by Sara when he was very young. Sara's sister took over trying to raise Cid, but when one of Cid's temper tantrums killed Sara's sister, Sara came back and is raising Cid. Sara is also telekinetic as minor telekinetic powers are present in some people in the future.
When Old Joe tries to kill the second child, Abe's men capture him. Old Joe is brought back to Cid's hideout, but escapes and in "Terminator" - fashion, kills Abe and his entire gang.
Old Joe shows up at the farm and chases Sara and Cid into a field with Young Joe following. When Old Joe grazes Cid's face with a gunshot, Cid's telekinetic powers threaten to engulf them all, but Sara calms Cid down.
Young Joe sees Sara standing between Cid and Old Joe. Young Joe envisions Cid's torturous young life and inevitable future to grow up as "The Rainmaker" if Sara is killed, so Young Joe shoots himself in the heart with his own Blunderbuss as Old Joe is about to shoot Sara. This causes Old Joe to immediately disappear as if he never existed at all.
We are left to presume that since Cid now grows up in a loving environment and Sara finds the stash of silver bars that Old Joe stole when he wiped out Abe and his gang, that Cid's destiny has been changed.
(Look for homages to other time travel movies including "12 Monkeys" and "The Terminator" and The Outer Limits episodes "Demon With a Glass Hand" and "The Man Who Was Never Born").
Thanks Evil Ed!