The title Full Moon, a reference to the lunacy a full moon is said to bring on, gives us a strong hint that this will be the classic Wodehouse love story that it is.
Love's mishaps in this instance occur at Lord Emsworth's Blandings Castle. First, Colonel Wedge and his wife, Lady Hermione, though initially dreading their visit to Blandings, are delighted when the wealthy young American heir Tipton Plimsoll, who is staying at Blandings, falls in love with their daughter, Veronica. As she is the "dumbest beauty" in Debrett's peerage, the parents have been wondering how they would marry her off.
Everything seems straightforwardly headed to a happy ending with this match when Tipton learns that Veronica was once engaged to Lord Emsworth's son, Freddie, the friend who brought Tipton to Blandings. Tipton starts to believe, wrongly, that Freddie is still secretly in love with Veronica, creating a series of comic events and misunderstandings that lead to Tipton and Veronica's breaking off their engagement.
In another love plot, the aristocratic Prudence Garland is engaged to the less eligible Billy Lister against the wishes of her family, which exiles her to Blandings to get her away from this unwanted suitor. However, despite Prudence's mother's objection to the match, as well as that of Lady Hermione, Billy shows up at Blandings in a series of disguises and continues to woo Prudence.
Finally, all is resolved happily by Galahad Threepwood, who manages to get both pairs of lovers reunited, which brings peace and an end to the "full moon" love lunacy at Blandings.