Jim meets the Wanderer in New Orleans as it arrives from Washington while two men in a carriage watch from nearby. When the agents comes onboard, he finds Rosa Montebello haranguing his partner, Artie, recently returned from his assignment in the nation's capital. Rosa angrily storms off and Jim discovers that Artie is secretly wearing earplugs. Once he removes them, Artie explains that Rosa is the niece of the Italian Minister and a famous diva in her homeland. She will be performing the title role in Lucia di Lammermoor at the New Orleans Civic Opera House and President Grant "suggested" that Artie accompany her to the city in the Wanderer. Rosa spent the entire trip complaining about the accommodations to Artie and now the President wants Artie to serve as her official escort during her three days in New Orleans.
Outside Rosa complains to the press and then boards her carriage. The two men are still watching and one of them, Igor, tells his golden-masked employer that all of the arrangements have been made for Rosa at the opera house.
As Artie regales Jim with his Washington adventures, a messenger brings a wire for Artie from Governor Turnball. Artie made the acquaintance of Turnball's daughter Nancy in Washington and the governor wants the agent to attend a party in her honor. However, the party is that night at the same time as Rosa's premiere performance. Jim offers to escort Rosa while Artie goes to Turnball's party to avoid insulting the governor and Artie eagerly accepts.
That night, Jim watches from a booth as Rosa performed Lucia. He notices someone in a private booth across the opera house watching Rosa carefully from behind a curtain. In the booth, the masked man tells Igor to put their plan into motion. The curtain comes down abruptly on the performance and Jim realizes that something is wrong. He slams through the stage wall as Igor arrives and orders his men, disguised as extras, to abduct Rosa. Jim forces them to retreat with Igor, but one of them grabs a spear and throws it at another extra, Pierre Deluc. Jim goes to the dying man, who indicates the ring on his finger. The agent takes it and discovers that it bears a unique lightning-like sigil.
Jim returns to the Wanderer to meet with Colonel Richmond while Artie goes to put Rosa into protective custody. Richmond informs his agent that there is a coded message inside the ring, written in Italian and referring to something called the "Order of Lucia." Miss Ellen Collingwood, secretary to the Secret Service's New Orleans chief, arrives and informs them that the dead extra, Deluc, has been employed at the opera house for years. He supervised the wardrobe and worked as an extra in his spare time, and has no known friends or relatives. When they mentioned the Order of Lucia, Ellen comments that in the last five years, three divas who sung the role of Lucia have disappeared without a trace and their ghosts supposedly haunt the opera house. The secretary suggests that the Jim check at the Club Musique for potential Order members, and he asks for Artie's help. Richmond informs Jim and Ellen that the President has specifically ordered Artie to attend Rosa to a reception held by Karl and Max Crenshaw at Willow Manor. Ellen points out that there have been rumors about Karl Krenshaw for years, but nothing definite. When she worries that Artie might have problems with Rosa, Richmond tells her that the agent came up with a way to deal with the temperamental diva.
That night, Artie, disguised as Colonel Vladislaw of Hungary, accompanies Rosa to the reception. Rosa is unaware of the deception and thoroughly satisfied with Vladislaw's wit and manners. As they make the rounds, Igor watches them from the balcony.
Jim goes to the cub where a singer, Angelique, is performing for the audience. Once she finishes her set, Jim talks to the bartender about the Order of Lucia and shows him the ring. The bartender claims that none of the members are there and goes in the back, while Jim waits. Angelique comes over and asks Jim where he got the ring. When she finds out where, Angelique explains that Deluc was her uncle. She knows that Deluc was involved at something at the opera house, and asks Jim to meet her there at 10 that night where they can talk without being overheard. As Jim leaves, two street singers lure him into an alleyway and more thugs attack the agent. Jim easily disposes of them and rides off.
At Willow Manor, Artie tells the crowd anecdotes about his fake life while Max invites Rosa out to the garden. As Artie watches them go, he notices Igor the butler is wearing a Order ring. Igor goes upstairs with a tray and Artie secretly follows him. Meanwhile, Rosa asks Max about his family and he explains that his brother Karl has confined himself to the manor since suffering an unfortunate affliction. Dropping the subject, Max asks Rosa about herself and she's glad to speak at length.
Artie goes upstairs to Karl's room but discovers that the door is locked. He cuts open a hole in the stained glass fixture and peers inside. The masked Karl is watching Max and Rosa and complaining about how his brother always abandons him. Karl then turns to the wall where a shrine to opera star Caroline Mason is mounted. Next to it is a handbill advertising her performance as Lucia as the New Orleans opera house. When Karl tells Igor to summon his brother, Artie hastily puts the piece of glass back in the fixture and hides. However, when Igor comes out he jars the plug loose. He tries to open a nearby wardrobe but discovers that it's locked from the outside, and looks inside a suit of armor. Finding nothing, he goes on and Artie uses a tool to open the outside lock on the wardrobe and make his escape.
Jim arrives at the opera house at 10 and goes to the wardrobe room with Angelique. She explains that before her uncle became the wardrobe manager, he served as a member of Caroline Mason's personal entourage. The opera singer died in a hotel fire five years ago, and Deluc has spent most of his time in the wardrobe room ever since. Angelique is sure that something in the room had a powerful hold on Deluc but doesn't know what. They hear someone moving behind the stage door and go outside. More thugs jump Jim and by the time he defeats them, Angelique has fled into the night.
Jim meets Artie at the train the next morning and informs his partner that by the time he went back to the club, both Angelique and the bartender had disappeared. They figure that everything ties into Caroline's death but don't know how. Ellen arrives with the address of Angelique's boarding house and tells Artie that he's to take Rosa to a society bazaar that afternoon. Once Jim leaves, Ellen suggests that Artie confine Rosa to her hotel room. Artie admits that he plans to use Rosa as bait to lure the kidnappers out, much to Ellen's shock. When she protests, Artie points out that he has no other alternatives unless someone volunteers to take Rosa's place. Ellen realizes that he means her and he convinces her to do so against her better judgment.
Jim goes to the boarding house and discovers that someone has torn up the room and abducted Angelique. Among the debris Jim finds a handbill for Caroline Mason's last performance and an A drawn in the powder on the floor.
Artie escorts a veiled Ellen out of the hotel and into a waiting carriage. As they depart, Artie tells Ellen that the kidnappers will wait a while before making their move.
Jim goes back to the opera house and checks out the wardrobe room. There are several lockers and one is marked "A." It has a metal keyhole and a slot that fits the raised sigil on the ring. When Jim inserts the sigil, the door opens, revealing an elevator. When he goes in, a thug closes the door behind him and the elevator descends into the earth.
When the carriage goes off course, Artie realizes that the carriage driver is abducting them. He makes a commotion for their kidnapper's benefit and the driver pumps knockout gas into their cab. Before they pass out, Artie assures Ellen that everything is going according to plan.
Jim finds himself in the catacombs beneath the opera house, and follows the sound of someone singing the part of Lucia to an air vent. He climbs inside and follows the shaft to a large chamber. Two missing divas are within giant birdcages, while the masked Karl Crenshaw plays the aria from Lucia and demands that one of the divas sing the part. Igor is there with his master and Angelique is chained to the wall. Karl stops playing and tells the diva that she is far inferior to Caroline Mason and that no one has sung that well before or since.
Max and his thugs bring in the veiled Ellen and the unconscious Artie, while Jim fires a dart-line across the chamber. Unaware of the deception, Karl tells Ellen that he will make her the singer that Caroline was or that Ellen will die trying. As Karl goes to the organ, Artie wakes up but pretends to still be unconscious. Karl orders Ellen to remove the veil so that she can sing, does it himself, and realizes that he's been tricked. Jim slides in on the dart-line and takes on several of the thugs, while Artie trips the others, grabs their guns, and holds them captive.
Ellen runs up the stairs and Karl goes after her. The secretary knocks Karl off the stairs where he falls 20' onto a table, shattering it. Jim goes to the dying man and removes the gold mask, an discovers that their opponent is actually... Caroline Mason. Max runs to her side as she dies and explains that he rescued Caroline from the hotel fire five years ago. A maid died in her place and Caroline's vocal chords were damaged by the smoke and fire. Caroline vowed to make herself whole again and give Max the voice he loved her for by training another singer to take her place.
Later, Rosa attends another reception in her honor. Jim and Ellen are there and Rosa comes over to tell them not to feel uncomfortable among such well-manned company. She makes sure that Artie isn't coming and then goes over to talk to "Count Vladislaw." The disguised Artie assures her that he would have challenged the Secret Service agent to a duel if he had come there, and the infatuated Rosa draws uncomfortably close to him.