“In Search of Cesium” is a book by Bulgarian writer Georgiy Markov, published in 1958. Plot: the famous Bulgarian biologist Kirill Rodovanov unexpectedly finds himself in a hospital on the verge of death. To save the professor, immediate brain surgery is needed, but it cannot be done without radioactive cesium preparations. This cesium is urgently delivered from the Soviet Union by plane, but when everything is ready for the operation, it turns out that someone has stolen the cesium wires straight from the operating room. Lieutenant Colonel Avroniev, who arrived at the scene of the incident, needs not only to identify the criminal, but also to find cesium within a few hours, otherwise the professor will die right on the operating table. The book tells about the exposure by state security agencies and ordinary people of Bulgaria of the criminal activities of foreign intelligence agents trying to disrupt the research work of Bulgarian scientists in the field of medicine and nuclear defense and to steal secret defense information.