In The Mardi Gras Mystery, Nancy's boyfriend, Ned Nickerson, is invited to spend the vacation with Brian Seaton, an Emerson College friend. On their way to the Seaton Mansion, Brian stops at Warren Tyler's house to pick up his father, Bartholomew Seaton, and at the same time shows Ned a portrait of his late mother, Danielle Seaton, by the famous artist Lucien Beaulieu. The painting is in the possession of Mr. Tyler since he found it in a barn he bought.
The friends leave for Seaton Mansion or "The Bat Hallow". They wear fancy dress for the Mardi Gras celebration. Later that evening they go to the Silver Yacht Club. That night the portrait is stolen. The prime suspect is Mr. Seaton, who is supposed to have wanted his wife's portrait. All the evidence points to him: he was wearing a bat costume, like the thief, and he was missing at the crucial time, around 10:00Â p.m.