Rebecca and Alec are in a hurry to prepare for their wedding. Mads agrees to be Rebecca's maid of honor, but Thayer is hesitant about being Alec's best man due to the suddenness of the marriage. Sutton shows Emma the picture of Ted in Rebecca's locket and tells her she should show it to Kristin.
When Kristin runs into Alec and Rebecca at the club, they tell her that they are getting married. After misunderstanding something Kristin says, Rebecca inadvertently lets Kristin know that something happened between her and Ted. Kristin leaves the club upset and runs into Emma, who shows her Rebecca's locket with the picture of Ted in it. This confirms Kristin's worst fear: Rebecca and Ted had an affair.
Sutton and Thayer meet with Dan and his girlfriend Theresa to figure out a way to find the truth. Ethan comes up with a plan to break into Rebecca's house during the engagement dinner to try to find proof that her alibi for Alec during the night of Derek's murder is bogus. Ethan and Sutton search through her files and find the picture of Alec holding the tire iron.
At the engagement dinner, Ted gives a speech about love, which sends Kristin barging off. When he chases after her, Kristin lets Ted know that she knows about his affair in L.A. Outside, Emma tells Laurel about the affair, and they leave with their mom.
Ethan shows Dan and Theresa the picture of Alec holding the murder weapon, but they tell him they can't use it because it was obtained illegally. Dan and Theresa say they need the actual murder weapon to prove Alec is guilty, so they get a search warrant. Meanwhile, Emma shows up and thanks Ethan for his help but also tells him that they are done.
After Alec leaves on the day of the wedding, Thayer helps Dan and the police search the house for the murder weapon, and they find tire iron in Alec's golf bag. At the wedding, Emma tries to confront Rebecca about the photo of Alec, but Rebecca says it's a fake. Thayer shows up just in time for the wedding, and the ceremony proceeds. Just after Alec kisses the bride, the police burst in and arrest Alec for Derek's murder.
In the aftermath, Rebecca slips away to her room, where Sutton waits for her. Sutton says, "Hi, mom." Rebecca responds, "It's all working perfectly, Sutton, just as we planned."