It began when the Mobile Devastator attacked the Sentai Golden City along with every single robot avaliable. the Mobile Devastator unloaded its cargo of Iron Drones and just began slaughtering hundreds of humans guarding the City. But Before it could do more damage, Ryo used his Battle Machine to create a high-pitched buzz that he filtered through his speakers, and disabled the Iron Drones, allowing the humans to do a lot of damage to the robots. Meca One retread, but the Mobile Devastator had acomplished half its goal when Meca One was able to capture and escape with Sensei Keiken, the leader of Exo-Force. After that, Hikaru, Takeshi, Ryo and Ha-Ya-To were given Jungle Battle Machines to go and rescue Sensei Keiken.
Exo-Force Online Comics 37-38
Exo-Force Book 5: Race to the Golden City (Inaccurate Canon)