The Second Mona Lisa is the one painted by Da Vinci, six months after the original.
O'Brien is a shady art dealer who has acquired it and has several interested parties willing to bid for it.
Colonel March has been called in to make sure everything is above board.
However March is aware that another art expert has been known to own the second Mona Lisa, Matthew Constable.
Before the bidding takes place. Constable has been brought into authenticate O'Brien's painting which means his would be a copy.
Texas oil magnate Wyatt wins the bid but later he thinks that the original painting has been switched with the copy.
The story ends up like a three card monte as March tries to find the lady with all the switcheroos.
There are no murders or even a queer mystery. The art game is more of a fraud game in this so-so story.