A preview promo broadcast by Lifetime later Wednesday night teases "toxic accusations," "scandalous behavior" and "shocking revelations" during the reunion special, which was filmed five months after "Decision Day" took place.
And Derek kicked things off with a jaw-dropping allegation.
"Katie confesses to me how she had an affair," Derek reveals in the preview.
However, a clip is then shown of Katie insisting, "That's not true."
On Married at First Sight, Katie admitted she was struggling to develop an emotional connection with Derek and she worried they just had a friendship.
Katie also often accused Derek of not making moves or initiating physical intimacy, which was a big problem for the couple since Katie said she likes when a man takes control in the bedroom.