 Virologist Dr Anna Hastings creates a virus, R.D.7, which she believes will wipe out rats. But after a lab assistant is bitten she is ordered to halt her work, and determined to finish her work takes to a remote boathouse in the Essex marshes to finish it. But when she too is infected by the virus, she is forced to ask the shady Patrice Constantine for help, and discovers that the infection might have begun to spread...