Epidemiologist Dr. Linus, A.P. photojournalist Jack Mann and E.R. nurse Annalisse try to stop a demonic entity that's possessing people from unleashing a biological weapon during the Saint Patrick's Day parade.

Jack Mann is a writer reporter for the Associated Press. Annalisse Summer is an ER nurse dedicated to help people: Dr Linus is a doctor who has seen what Jack and Analisse have: an evil entity that seeks to destroy and has joined them to help stop it (the entity posses people and controls them to do its bidding).—Vincent Terrace

Professor of epidemiology Doctor Linus is called to Boston on Saint Patrick's Day to help virologist Doctor Cornelius investigate the mysterious death of a woman seemingly killed in a hit and run accident. Despite his editor Rita's protest, Associated Press photojournalist Jack Mann is also there to investigate why the dead woman's sister and dedicated emergency room nurse Annalisse Summer has inexplicably fallen into a catatonic state during the E.R. staff's attempt to revive the woman. Jack actually has a very personal reason why he's interested in the case. Aging mysterious demon hunter Aaron Rainier approaches both Linus and Jack to warn them that what their dealing with is a case of demonic possession, and he asks them both to join him in his attempt to defeat the demonic entity that has taken over Annalisse. Meanwhile, with the help of a non-possessed evil accomplice, the possessed Annalisse awakens and attempts to steal a vile of flesh eating virus that was developed in the hospital's secret wing by the military in their top secret pursuit to create the ultimate biological weapon.