In an alternate modern America, Lily Mossberg, an anthropology lecturer at a rural New England university, has the ability to turn herself into a tiger. This ability is due to a sudden rash of genetic mutations, a "psi plague", of which shape-shifting is a rare and virtually unstudied variant. Lily must contend with her ex-husband, the powerful head of a pharmaceutical company, who is ignorant of her ability but who wants to make money off the new psi "diseases". He also wants custody of their apparently normal nine year old daughter. The government has a public health mandate to eliminate the mutations. Lily must struggle to keep her secret and save herself and her daughter from these competing interests even as the realization grows that there may be a sinister collaboration between them. Then there are the murders, which do not have an entirely human signature. An investigator from the federal Psi Special Unit is called in. Lily finds herself both investigating and investigated, determined not to fall in love again but drawn to the detective who is supposed to investigate the murders. Things come to a head at her ex-husband's secret pharmaceutical facility where more than Lily's life and that of her daughter are at stake.