3 ideas
Joseph Keeble cannot spend his own money because his wife, Lady Constance, controls the household finances. His nephew Freddie Threepwood answers the newspaper advertisement placed by Psmith, who has announced that he will do anything, and Psmith takes the job and gets into Blandings Castle by impersonating the Canadian poet Ralston McTodd.
Psmith passes as the poet McTodd because no one at Blandings has ever seen McTodd, so the only test anyone applies is whether he behaves the way a poet is expected to. Confident manner and fluent talk fill the gap left by that missing reference point.
Rupert Baxter, the efficient secretary, is right that impostors and thieves are in the castle. Locked out at night in his pyjamas, he throws flowerpots at Lord Emsworth's window to get attention, and Emsworth reads this as proof of madness. Baxter is dismissed for looking unhinged, while the smooth Psmith, who really is an impostor, gets his job, showing that people judge credibility by composure rather than by accuracy.