Mark is ducking Inspector Swan, but is captured. The SID Inspector shoots him away on a giant arrow. Mark survives and the others find Swan, dead. He has been killed with a curare dart to the ankle, and his murder is apparently the second to have occurred. The first was a carnival operator who held the Earthlings captive in the past.
Reporter Bertha Fry is stirring up trouble and turning public sentiment against the Earthlings, much to the disgruntlement of Lt. Grayson, head of the investigation. A miniature blowpipe with the initials MW on it point to Mark as the killer, and even some of the Earthlings begin to wonder. Mark comes back to camp, denies everything, and then leaves after a brief brawl. An explosion occurs in the city and explosives are missing from the ship.
Dan and Steve sneak into SID HQ, where they overhear Grayson talking to Zoral (from the episode "The Mechanical Man"), who was assaulted. The two men are caught but manage to escape, and witness Zoral being killed with another curare dart. Mark eventually manages to send a weak signal. Ducking a magnet trap left by the late Swan, the Earthlings track Mark's signal to SID HQ!
It turns out that Grayson's subordinate, Barker, has captured Mark (and also captures Fitzhugh and Dan) and is framing the Earthlings for the murders to discredit Grayson. Barker tries to kill Grayson with another curare dart, but Steve warns the lieutenant and after a brief struggle, Barker is knocked out. Barker cheerfully confesses to everything, and a reluctant Grayson lets the Earthlings go because of their warning that saved his life.