The Hidden Enemy " The Hidden Enemy " is the sixteenth episode of of the " " television series . It is the sixteenth episode of the series overall . It serves as a prequel to , and chronologically falls between the episode " Cat and Mouse " and the film . Official description . " Anakin and Obi - Wan discover evidence of a traitor amongst their ranks while attempting to lead Republic forces to liberate the planet of Christophsis from a Separatist siege . When the Republic is ambushed on Christophsis , it becomes clear that someone has set them up . The Jedi suspect the Sith , while Rex and Cody uncover information that points to a spy amongst them . " Plot summary . Anakin , Obi - Wan , and their clones prepare to ambush the advancing droid forces from two towers . Just as they are about to fire on the droids , the advances split into three columns and droids suddenly appear in Obi - Wan ' s building . Anakin calls in a gunship for immediate evacuation before leading his squad to assist Obi - Wan . They escape to the roof , and the droids follow them . Their gunship arrives , picking them up , but not before taking the command tactical droid ' s head to find out how the Separatists discovered their plan . At the base , Rex and Cody analyze the droid ' s memory , but the best they can find before the head malfunctions is that the Separatists discovered their position ahead of time . Obi - Wan suggests that they may have a spy in their ranks . The Jedi decide to go behind enemy lines to investigate and tell the clones to keep the mission classified . Rex and Cody then sweep out of the base , but as the Jedi leave , Cody notices that someone left his comlink on , eavesdropping on them . They spot someone running off and chase him . Unfortunately , they lose him in the mess hall , among their many brothers , meaning that one of the clones is a traitor . They decide to contact the Jedi , but the droids are keeping the communications jammed . Rex and Cody have Artoo analyze the communication history , to find out if anyone had sent out any unauthorized transmissions . Rex notices an irregular wavelength that is too low and infrequent for them to normally detect . The transmission seems to be coming from Slick ' s barracks , so they decide to investigate . They question all of Slick ' s men : Jester was cleaning his weapon , Punch and Sketch were in the mess hall , and Gus was in the infirmary . The last one , Chopper , appears that he is hiding something , and reluctantly reveals that he was collecting droid fingers , which is forbidden by protocol . Chopper denies being a spy , and Slick offers a full investigation when the Jedi come back . However , Chopper snaps that from his hiding place , he saw Slick turn into the command center rather than the mess hall . Cody also notes that Slick somehow knows about the Jedi ' s departure , while only Rex and Cody knew about it . Slick , knowing he ' s been discovered , punches Cody and Chopper , and escapes . Cody and Rex try tracking him , but they are too late : Slick blows up many of the gunships and AT - TEs , as well as the weapons depot , with earlier - placed charges . The other clones inform Cody and Rex that Slick went into the command center , and they order the others to secure the perimeter while they go inside . In the meantime , as Anakin and Obi - Wan race to the Separatist base on BARC speeder s , they notice an increasing number of droids spying on them , but still decide to continue the mission , despite the fact that it has been compromised . They arrive at the enemy base and inside , they meet Asajj Ventress again and begin to duel with her . The Jedi manage to overcome Ventress and demand her surrender , but as they edge closer to capture her , she creates a large hole in the floor beneath their feet with her lightsaber s , pushing them one level down . Ventress taunts them and runs away , and the Jedi follow her , discovering that she has set up a whole army to invade Christophsis . The Jedi engage Ventress again , overbalancing the tri - droid she is standing on before escaping on STAPs . Ventress dispatches General Loathsom to take care of the Jedi while she moves on to the next phase of her master ' s plan , and she heads to the " Trident " . Rex and Cody enter the command center in search of Slick . They soon guess that he is hiding in the vents and inconspicuously act out a scene ; Cody leaves his blaster on the table while Rex goes to " secure the lockdown " . Slick comes down from the vents and grabs Cody ' s blaster , only to find that it is empty , and Rex comes up from behind him and puts his blaster at Slick ' s head . Slick engages the commander and captain in physical combat , stating that he was tired of all the suffering and was offered freedom and money from Ventress . Cody manages to knock him out and they apprehended him . As they present Slick before the Jedi , Slick snaps that his brothers are enslaved by the Jedi , that he was striking a blow for all clones and that he loves his brothers , but Cody and Rex retort that he has now exposed them all to certain doom . Cody orders the other clones to take the traitor to lockup . Obi - Wan asks if they managed to salvage anything from the weapons depot . Rex says that though most of their weapons are trashed , they managed to save the heavy cannons , which will prove to be useful in the coming battle . Behind the scenes . The script originally dictated the start of the episode to involve the clones setting up a combat droid nicknamed " The Beast , " similar in design to the dome - shaped mining droids seen in " " . However , to keep the war defined as clones vs . droids and not clones using droids vs . droids , and since the sequence didn ' t even explain the droid ' s use , the sequence was cut . At the beginning of the episode , when Cody and Slick return , Cody is carrying a plain white clone helmet . He is seen holding his rightful helmet later on in the episode . When Obi - Wan is explaining the Jedi ' s mission behind enemy lines to Cody and Rex , he calls Cody " Captain " when Cody is clearly a Commander . The mistake arose during the editing process , since originally Obi - Wan was talking to Rex . Also , before Cody and Rex speak to Slick ' s men , Slick says , " Captain , if I can just have a word with them . . . " To this statement , Cody turns and looks at Slick . During the episode , Chopper ' s right eye changes color . When he is first cast under suspicion , his right matches his left eye ' s brown color . Later , however , his right eye is an off - white color . Much of the background music consisted of an edited version of the " " track from . Additionally , the music for the brawl between Rex , Cody and Slick is a variation of the speeder chase music from " " . " The Hidden Enemy " had aired on September 16 , 2012 on Atlanta ' s MyNetworkTV prior to a televised airing of " Revenge of the Sith " . When Kenobi , Skywalker , Cody , and Rex are examining the tactical droid ' s severed head , the word ' tactical ' is written in Aurebesh on the side of the droid ' s head .