The Reluctant Murderer

We know that Vivian Haines intends to commit murder this weekend. She tells us so. But who is her intended victim? Could it be her wealthy aunt, who is supposed to leave her half her fortune one day? Or her frivolous sister and her seemingly penniless boyfriend? Or perhaps her aunt's mousy companion, or her long-suffering chauffeur? Or Vivian's own fiance, the fastidious Cuthbert? All we know is what Vivian tells us as her efforts to plan and execute the perfect murder are constantly thwarted. Now Vivian is beginning to panic. Could one of them suspect her? Could one of them be planning to kill her before she can murder them first?