William Ballinger, 19, was arrested Friday by Shelbyville police, Detective Sgt. Brian Crews said.
Police are still searching for Billy Joe Colburn, who Crews said was behind the plot.
"Our investigation has revealed that Billy Joe Colburn set out to find a Hispanic male to rob," Crews said. "To facilitate the robbery Billy Joe Colburn solicited the help of William Ballinger and two females who were to pose as girls willing to prostitute. The females were willing participants in the plot."
McKenzie Drew Hickson, who Crews said has just turned 18, and a girl who just turned 17 have been arrested.