During her visit to Cabot Cove, Sibella appears to be a targeted victim of a murder attempt, as she fell through a floorboard into the tide while she was out walking. However, the mishap was part of Sibella's evil plan to kill Marian King, who was not only Sibella's assistant, but the mistress of Sibella's husband, Derek Hartman. Sibella sawed the floorboards herself to cause the fall, and she manipulated her illicit lover, Monroe Shepard, to continue the deception--as he wrote a threatening note to Sibella and signed it "JW," the initials of Joseph Wallinsky--Sibella's fugitive ex-husband.
The scheming villainess began setting up Marian's murder by hiding her own coat so Marian would have to take Sibella's white cloak on her way out. After Marian left, the evil Sibella donned a hooded disguise and followed her intended victim to the boathouse, where she struck Marian with a crowbar, killing her instantly and knocking her over. Marian wearing Sibella's cloak part of Sibella's plan to make it appear that Sibella herself was the intended victim, and Marian was killed as a case of mistaken identity.
RevealEditSibella was revealed as the villainess right after Monroe was revealed as a conspirator, as he met Sibella at the boathouse discussing Derek knowing about their affair, while pinning Marian's murder on Derek. When she heard Derek coming, Sibella began turning heel and pointed a gun at Monroe, acting like she was being held captive by him. After Derek arrived, though, Sibella fully turned heel and pointed her gun at her husband, with Monroe realizing that Sibella killed Marian and was planning to kill Derek as well. She was stopped by Jessica and Sheriff Metzger, and following the reveal, Jessica pointed out that Monroe's misspelling of "mince" led to him being revealed as a conspirator and also outed Sibella.
Derek suggested that Sibella's motive was his affair with Marian, but Jessica stated that may have been part of it; Sibella was also enraged that Marian was looking to become an author, claiming that Marian had used her. The aforementioned note was to make it appear that her ex-husband was after her, and regarding the attempt to kill Derek, Sibella had planned to kill him with one gun, while killing Monroe with a second gun, while planting the first gun on Monroe to make it appear that he killed Derek and she had to kill Monroe in self-defense. Sibella was arrested (off-screen) for killing Marian and the attempts on her husband and Monroe.