with blood in the back seat and a knife made of an alloy that has not yet been invented on Earth. The Doctor soon determines that the Tracer is picking up another source of chronon radiation which is not related to the Key to Time, but since he can't locate the fourth Segment until the interference is dealt with, he and Romana must remain to investigate. They trace the radiation source to Gongpinglu Wharf, where Li is about to move in on Kwok and Hsien-Ko as they purchase smuggled German weapons from Vogler. Their unexpected arrival startles Li into ordering his team into action prematurely, and Hsien-Ko and Kwok thus escape, killing Vogler to prevent him from identifying them. Li arrests the Doctor and Romana, and when the Doctor's fingerprints prove to match some found on the abandoned car, he charges them with the murder of the man from the opium den. The dead man has now been positively identified as the caretaker of the Palace Museum in London—which is impossible, as he was spotted alive in London one hour before his body was found in Shanghai. The Doctor summons K9 for help, and K9 arrives just in time to save them from bandits who materialize out of thin air in the police cells and attempt to kill them. The Doctor studies the stunned bandits and finds that they are all footsoldiers from the Tong of the Black Scorpion and that they are carrying geomantic compasses embossed with the blood of a young woman. The Doctor modifies K9 to seek the signature of the chronon radiation he is seeking, and K9 leads him and Romana to the opium den, just as Kwok arrives to retrieve the Tong's abandoned altar and Li arrives in search of more clues. The Doctor and Romana steal a car to pursue Kwok, and Li sets off in pursuit of them all. Kwok uses his geomantic compass to transport his truck elsewhere instantaneously, but the pursuing cars are swept up in the currents behind him and the chase continues, until a Japanese bomber strafes the city and the Doctor's "borrowed" car is buried under the rubble. Li pulls the Doctor out of the wreckage and arrests him, leaving Romana for dead, but Romana is rescued by Woo and taken back to his nightclub to recover. Woo explains that as a vigilante he protects the ordinary people