Phryne is sitting in the lobby of a Paris hotel reading the newspaper when she hears a distressed Australian woman begging the hotel concierge for help. Phryne's partner Alain Descourt is concerned they will miss the Ring of Nibelungs but Phryne goes to see what the fuss is about. She asks the concierge Jean-Paul what is happening and she says the women had been there twice that night asking about her husband but there was no record of him in the register and they did not have their passports which is the law.
Phryne asks the woman Mrs Johnson what has happened and she says the hotel have stolen her husband. Phryne takes the woman into the drawing room of the hotel and orders coffee and cognac and once the woman has calmed down asks for the details. Mrs Johnson said they came of the Orient and took the train to Paris then took a taxi to the hotel and her husband Arthur signed in. The concierge took their passports and showed them to room 311A. Where they set down their things and bathed. Mrs Johnson said her husband said he felt sick so she asked the manager to phone a doctor and the doctor came and told her to get some medicine from a pharmacy but they didnt have the medicine so she went from pharmacy to pharmacy on foot as she had no money for a taxi. She had been gone two hours before returning to the hotel.
Phyrne asks her if she still had the prescription but Mrs Johnson said the second chemist kept it as he said he'd send the medicine on. Phryne checks that Mrs Johnson is back at the right Hotel Splendide as it is a common name but Mrs Johnson says she recognises the décor and the clerk but when she returned to the clerk he said he had never seen her before and there was no room 311A. Mrs Johnson says she asked to be shown and there was just a bare wall where the room once was. Mrs Johnson says the trip had been their honeymoon and they had been to Egypt, India and Ceylon.
Phryne leaves Mrs Johnson with Alain and goes to talk to the clerk. He offers to show her the register and Phryne notes it is new which is strange as she was sure there were many pages left in the old register when she signed in. The clerk also takes her up to show her there is no room 311A. Phryne bends down to inspect the wall drawing her glove across it.
Phryne returned to the withdrawing room and asks Mrs Johnson if she knows the name of the doctor. Mrs Johnson says it was Dupont which is a common surname. Phryne then hold her glove to the lady's face and asks what she can smell. Mrs Johnson says Jicky, rice powder and wallpaper paste. Phryne asks the woman to trust her and ask Alain to find the nearest doctor and bring him to the hotel. She then tells the concierge that she and Mrs Johnson are going to rest in her suite.
Mrs Johnson suppers with Phryne and bathes. Alain returns with his doctor friend Lestrange and Phryne tells him she is going to do something illegal and needs him to watch on. They inquire what illegal act she is going to do and she says she is going to set a fire in the hotel and to come when they smell smoke. The smell smoke and go with Mrs Johnson to Phryne. They find her attacking the wall with a poker and go to help. They find a door beneath the freshly wallpapered wall behind the door is the Johnson's room with Mr Johnson sick in the bed. Alain asks why they would do this and Phryne says the hotel would have been in fear of the plague from Bombay. But Mrs Johnson said they never stopped at Bombay. Phryne asks Dr Lestrange what is wrong with the patient and he says Mr Johnson has malaria.
John-Paul confront Phryne about what she has been doing and she says the found Mr Johnson sealed in his room and that he had malaria not the plague. John-Paul says he is sorry and takes Mr Johnson to the royal suite and then arranges the medicines Dr Lestrange requests.
Then Phryne takes Jean-Paul for an explanation. Jean-Paul says he was in India when the plague hit and it killed his brother. He feared Mr Johnson had it so he called his cousin the doctor and sent Mrs Johnson for medicine. Phryne asked if they were going to remove Mr Johnson or just wall him up alive and Jean-Paul says he was almost dead already. Phryne decides she will not report him provided he let the Johnsons stay free of charge until Mr Johnson is well and send a friend to show Mrs Johnson the sites of Paris including a complete spring costume from one of the fashion houses. Jean-Paul agrees and Phryne and Alain get to the opera just in time to see the last act.