Bertie returns to London from several weeks in Cannes spent in the company of his Aunt Dahlia Travers and her daughter Angela. In Bertie's absence, Jeeves has been advising Bertie's old school friend, Gussie Fink-Nottle, who is in love with a goofy, sentimental, whimsical, childish girl named Madeline Bassett. Gussie, a shy teetotaler with a passion for newts and a face like a fish, is too timid to speak to her. Bertie is annoyed that his friends consider Jeeves more intelligent than Bertie, and he takes Gussie's case in hand, ordering Jeeves not to offer any more advice.
Madeline, a friend of Bertie's cousin Angela, is staying at Brinkley Court (country seat of Aunt Dahlia and Uncle Tom). Aunt Dahlia demands that Bertie come to Brinkley Court to make a speech and present the school prizes to students at the local grammar school, which he considers a fearsome task. Bertie sends Gussie to Brinkley Court in his place, so that Gussie will have the chance to woo Madeline there, but also so that Gussie will be forced to take on the unpleasant job of distributing the school prizes.
When Angela breaks her engagement to the athletic but heavy Tuppy Glossop, Bertie feels obliged to go down to Brinkley Court to comfort Aunt Dahlia. In addition to her worry about Angela's broken engagement, Aunt Dahlia is anxious because she has lost 500 pounds gambling at Cannes, and now needs to ask her miserly husband Tom to replace the money in order to keep financing her magazine, Milady's Boudoir. Bertie advises her to arouse Uncle Tom's concern for her by pretending to have lost her appetite through worry. He offers similar advice to Tuppy, to win back Angela. He also offers the same advice to Gussie, to show his love for Madeline. All take his advice, and the resulting return of plates of untasted food upsets Aunt Dahlia's temperamental prized chef Anatole, who gives notice to quit. Not unreasonably, Aunt Dahlia blames Bertie for this disaster.
When Bertie attempts to probe Madeline's feelings about Gussie, she misinterprets his questioning as a marriage proposal on his own behalf. To his relief, she tells Bertie she cannot marry him, as she has fallen in love with Gussie. Bertie relays the good news to Gussie, but even with this encouragement, Gussie remains too timid to propose, and Bertie decides to embolden him by lacing his orange juice with liquor.
Gussie ends up imbibing more liquor than Bertie had intended. Under its influence, Gussie successfully proposes to Madeline. He then delivers a hilarious, abusive, drunken speech to the grammar school while presenting the school prizes. Madeline, disgusted, breaks the engagement and resolves to marry Bertie instead. The prospect of spending his life with the drippy Madeline terrifies Bertie, but his personal code of chivalrous behavior will not allow him to insult her by withdrawing his "proposal" and turning her down. Meanwhile, Gussie, still drunk, retaliates against Madeline by proposing to Angela, who accepts him in order to score off Tuppy. Tuppy's jealousy is aroused and he chases Gussie all around the mansion, vowing to beat him within an inch of his life.
In the face of this chaos, Bertie admits his inability to cope, and appeals to Jeeves for advice. Jeeves arranges for Bertie to be absent for a few hours, and during that time swiftly and ingeniously solves all the problems, assuring that Angela and Tuppy are reconciled, that Gussie and Madeline become engaged again, that Anatole withdraws his resignation, and that Uncle Tom writes Aunt Dahlia a check for 500 pounds. Bertie learns his lesson and resolves to let Jeeves have his way in the future.
Sections of the story were adapted into episodes of the ITV series Jeeves and Wooster.

Question: Why does Madeline tell Bertie that she cannot marry him?
Answer: Because she is in love with Gussie
Alternative answer: She's in love with Gussie.