The first-person narrative is told from the point of view of Alexei Ivanovich, a tutor working for a Russian family living in a suite at a German hotel. The patriarch of the family, The General, is indebted to the Frenchman de Criet and has mortgaged his property in Russia to pay only a small amount of his debt. Upon learning of the illness of his wealthy aunt, "Grandmother", he sends streams of telegrams to Moscow and awaits the news of her demise. His expected inheritance will pay his debts and gain Mademoiselle Blanche de Cominges's hand in marriage.
Alexei is hopelessly in love with Polina, the General's stepdaughter, and swears an oath of servitude to her. He told her while on a walk on the Schlangenberg (a mountain in the German town) that all she had to do was give the word and he would gladly walk off the edge and plummet to his death. This leads to her asking him to go to the town's casino and place a bet for her. He refuses at first but, when goaded and reminded of his oath of undying love and servility, he succumbs and ends up winning at the roulette table. He returns to her the winnings but she will not tell him the reason she needs money. She only laughs in his face (as she does when he professes his love) and treats him with cold indifference, if not downright malice. He only learns the details of the General's and Polina's financial state later in the story through his long-time acquaintance, Mr. Astley. Astley is a shy Englishman who seems to share Alexei's fondness of Polina. He comes from English nobility and has a good deal of money.
One day while Polina and Alexei are on a walk they see Baron and Baroness Wurmerhelm. Polina dares him to insult the aristocratic couple and he does so with little hesitation. This sets off a chain of events that details Mademoiselle Blanche's interest in the General and gets Alexei fired as tutor of the General's children. Shortly after this, Grandmother shows up and surprises the whole party of debtors and indebted. She tells them all that she knows all about the General's debt and why the Frenchman and woman are waiting around the suite day after day. She leaves the party of death-profiteers by saying that none of them are getting any of her money. She then asks Alexei to be her guide around the town famous for its healing waters and infamous for its casino where the tables are stacked with piles of gold; she wants to gamble.
After being ushered to the roulette table, she plays and wins 13,000 Friedrich's d'ors (7000-8000 roubles), a significant amount of money. After a short return to the hotel, she comes back to roulette tables and she starts to get the bug; before she leaves the town, she's lost over a hundred thousand roubles in three days.
When Alexei gets back to his room after sending Grandmother off at the railway station, he's greeted by Polina. She shows him a letter where de Criet says he has started legal proceedings to sell Generals' properties mortgaged to him, but he is returning properties worth fifty thousand roubles to General for Polina's benefit. de Criet says he feels he had fulfilled all his obligations that way. Polina tells Alexei she is de Criet's mistress and she wishes she had fifty thousand to fling at de Criet's face. Upon hearing this, Alexei runs out of the room and to the casino where he in a feverish rush of excitement wins in few hours two hundred thousand florins (100 000 francs) and becomes a rich man. When he gets back to his room and the waiting Polina, he empties his pockets full of gold (Alexei estimates the weight to some 4 kilos) and bank notes onto the bed. At first she accuses him of trying to buy her like de Criet, but then she embraces him. They fall asleep on the couch. Next day, she asks for fifty thousand roubles (25 000 francs) and when he gives it to her, she flings that money at Alexei's face and runs off to Mr. Astley (they had been secretly meeting and exchanging notes and she was supposed to meet him night before but has come by mistake to Alexei's room). He doesn't see her again.
After learning that the General wouldn't be getting his inheritance and that Prince Nilski is penniless, Mademoiselle Blanche leaves the hotel with her mother/chaperone for Paris and seduces Alexei to follow her. Alexei goes with them, and they stay together for almost a month, he allowing Mlle Blanche to spend his entire fortune on Mlle Blanche's personal expenses, carriages and horses, dinner dances and a wedding-party. After getting herself financially secured, in order to get an accepted status in the societies, Mlle Blanche unexpectedly marries the General, who has followed her to Paris.
Alexei starts to gamble to survive. One day he passes Mr. Astley on a park bench in Bad Homburg and has a talk with him. He finds out from Astley that Polina is in Switzerland and actually does love him. Astley tells that Grandmother has died and left Polina and the children financially secured. The General has died in Paris. Astley gives him some money but shows little hope that he will not use it for gambling. Alexei goes home dreaming of going to Switzerland the next day and recollects what made him win at the roulette tables in the past.
Output: What oath does Alexei plead to Polina?