The incident and climax is that the murder plot that Mrs. Stevenson overhears actually involves her and was engineered by her husband. Mrs. Stevenson herself gives clues when she says she lives by a bridge with a policeman walking a beat. By the time she figures it out, it is too late.
My suggestion, if you haven't already done so, is listen to the play as performed by Agnes Moorehead. The easiest place to find a downloaded version of the play is to go to www.escape-suspense.com and download one of the seven versions available to hear. Moorehead originated the role of Mrs. Stevenson and is magnificent in the role as she performs it in all seven versions.