We're Planning a June Wedding

Matt's mother, Kelly returns to Mystic Falls claiming to want to be there for Matt, but it is soon revealed that she is working with Katherine and had been residing in Hell after having died two years ago. Damon concocts a plan to lure out Katherine by throwing a last-minute wedding of Stefan and Caroline. Damon and Stefan also construct a dagger made from Katherine's bones to kill her. Bonnie tells Caroline she can't be at the wedding of the man who killed Enzo, but Enzo encourages her to go for her best friend. At the wedding, Kelly slits Peter's throat after a failed attempt to get the dagger from him. Everyone expects Katherine to interrupt the ceremony, but she doesn't, so Stefan and Caroline say their vows and actually get married. During the reception, Bonnie goes with the twins to the Lockwood house; Kelly interrupts Damon's speech and reveals that the Lockwood house is going to catch fire because she started a gas leak. The fire starts with Bonnie and the girls in the house. However, Lizzie and Josie are able to siphon Bonnie's magic to save them and they all use their power to escape with Bonnie having to lose her connection to Enzo in the process. In the end, Kelly reveals that Katherine's plan was to distract them with the house fire while the now resurrected Vicki Donovan returns and begins ringing the Maxwell bell to bring hellfire to Mystic Falls. As the bell rings, Bonnie falls to the ground, nose bleeding, and presumably dying.

Plot summary from Wikipedia (CC BY-SA 4.0). Source