Cal Rigdon, an old friend of Hood's asks him to come to Philadelphia to help out with an outbreak of a mysterious infectious disease. Rigdon heads up the Pennsylvania Health Dept. and a local demolition worker exhibits symptoms of a disease which could have profound world health consequences unless they identify and contain the pathogen. As Hood and Rachel investigate, they find the original victim -- a mortuary worker who came to work at the demolition site -- and then the source of the infection. When a local cold storage facility had compressor problems, samples of a particularly lethal form of smallpox virus which were being stored illegally in the facility thawed, exposing the repairman.
Hood and Rachel track down the complex pattern of how the repairman's exposure lead to the demolition workers infection and find the facility. Some of the workers exhibit similar symptoms and Rachel risks her life in order to stop them from sneaking away and unwittingly spreading the virus. Unfortunately, she gets exposed and must also be quarantined. Hood finds who was storing the virus and manages to contain it before it spreads any further.