Jen Aniston witnesses Ashton Kutcher wail on heckler who disses the missus Demi Moore.

Earlier that night The Kutch was all smiles following his B’way debut in Montblanc’s 9th Annual "24 Hour Plays."

But things turned ugly at the after party at Crest Lounge’s Opera Ballroom when according to an eyewitness a party goer mocked Ashton’s costar and wife Demi enraging the 31 year old!

"Demi and Jennifer Aniston were happily sitting together in the VIP when this guy starting really obviously making fun of Demi and things like her relationship with a younger man," an insider told FoxNews.com

 "It went on for quite a while. Ashton heard it and approached him. At first he thought the guy wasn’t serious and was like ‘what the?’

"He then realized he was serious. Ashton lost it."

According to Fox, a "massive verbal fight then ensued between Ashton and the guest, as Demi, 47, looked very embarrassed over the whole situation."

"It got pretty heated nobody knew what to do," the insider said as well.

The couple, with Jen trailing, then stormed out.