Sirens.

Ali Pearson is an English law officer simultaneously involved in a heated and clandestine relationship with her sister's beau and a city-wide police hunt for a serial rapist. When the squads close in on the boyfriend as their prime suspect, it foretells Ali's need to 'lay down her cards' by confessing her relationship to the man in question. But this may mean permanently losing all ties with her sister. Ali suddenly finds herself torn violently in two directions - but she's quickly running out of time, as new details come to light regarding the suspect's background and motivations.