When Rick and Kate go to get their marriage license in Castle Season 6 Episode 23, they find out that Kate is already married to her college boyfriend Rogan O'Leary.
They took a trip to Vegas and she thought the Drive-thru chapel was a joke. It wasn't and now she needs Rogan's signature to disolve the marriage.
Castle and Beckett get pulled into Rogan's blackmail scheme and have to save his life from a mobster in order to obtain his signature.
Back in the city, due to a fire and a burst water pipe, they lose both their wedding venue and Kate's dress but Martha moves the entire wedding out to the Hamptons and Jim Beckett brings Johanna's wedding gown for Kate.
As Castle drives to the wedding, a black SUV rolls up aside his car. Kate gets a phone call and runs out of the wedding only to find Castle's car in flames on the side of the road.
If you need to see the drama unfold you can watch Castle online here at TV Fanatic: