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For four years, delinquents who go by the collective name of Fence Jumpers have been vandalizing the good city of Cincinnati, in the province of Ohio. They leap over fences that separate private property from public, and government buildings from unauthorized personnel. These fences don't deter the Jumpers, and the buildings are generously painted.
But this mask of vandalism and delinquency hides their true mission. The Fence Jumpers suspect questionable activities with the higher-ups, and they make it their job to find out what. They suspect the authorities of creating cyborgs: half-human, half-robot super soldiers.
When Skyler Pelton and his sister Hannah take shelter with the Jumpers after almost being caught by Nabbers, people who make irregular sweeps and kidnap the homeless or orphaned off the streets for unknown reasons, they find a home with new friends who become like family. They're trained to become Jumpers, and get a few opportunities to make their start as a juvenile mischief-maker.
But things aren't quiet and calm for long. The Jumpers dive even deeper into the dark mystery of the cyborgs. Things get crazier when freelance journalist and photographer Madison Carter becomes an ally above ground, after being kidnapped by the Jumpers for photographs she took that could expose the identities of some of the Jumpers. A possible traitor in their midst and the loss of a young Jumper during a scouting mission gone wrong makes everyone nervous. Tempers are short and easily triggered. Skyler struggles with making sense of it all, especially when everything that goes wrong seems to involve him.
But then the worst happens: the Nabbers find the underground hideout. Skyler and a handful of Jumpers escape, but Skyler feels that he's lost more than a home. He watched Hannah get shot, and had been unable to protect her. After taking shelter at Madison's apartment, Cody, a boy who had been like Skyler's mentor and an older brother, takes Skyler and a few others on a reckless rescue mission. Joined later by Kip, the only adult among the teens, and more Jumpers, they enact their rescue mission. Jumpers are freed, and Wes and Hannah, both presumed dead, are found alive. The Jumpers scatter to later regroup outside the city, but Skyler is taken captive.
Skyler meets Horatio Vernalti, mayor of Cincinnati and the mastermind behind the cyborgs. Horatio's greedy desire for power fueled the manufacturing of cyborgs. Skyler also discovers that Claire, who had been a Jumper and friend, is the traitor, as well as Horatio's niece. Skyler is tortured for information on where his friends are, but his ignorance doesn't save him.
Cody and Kip go back to look for Skyler and learn of his capture. They pounce on the only chance of a rescue, and succeed, but only at the sacrifice of Cody's life. Skyler and Kip escape Horatio's clutches while Cody, having been injured too much to run, buys them time inside Horatio's underground laboratory.
Skyler is reunited with Hannah, and the Jumpers prepare to hop the incoming train to escape the city forever. However, Nabbers show up and begin taking down the fugitives, including Kip. Skyler and a very small number of Jumpers escape, and they learn that Grace had a tracking device embedded in her shoulder. After some deliberation, everyone but Grace jumps off the train to go back and rescue their friends.
Now, Skyler and his friends face the biggest challenge than any fence has ever given them. And it might just become a war...