The book begins, and most of the action takes place, on land and follows the financial and romantic dealings of the two main characters Jack Aubrey and Stephen Maturin. It introduces the two main female protagonists of the series, Sophie Williams and Diana Villiers, and we get to see O'Brian develop them into familiar, believable characters. He achieves some success with this in Post Captain, the character of Diana in particular being well developed with all facets of her personality, both admirable and not so much, leaping off the page at us. It must be said that in later novels O'Brian will struggle to portray his female characters as well as he does here - although his treatment of the many, many male characters will remain outstanding - and, although Diana will continue to develop right up to her untimely death in a later novel (and I will have much, much more to say on that when I get that far into the series), Sophie seems to become more and more one dimensional as time goes on.
There are sufficient passages set at sea to remind us that this is, in fact, nautical fiction and O'Brian's skill in this genre comes to the fore. Particularly impressive is his dealing with the sailing qualities (or lack thereof) of the Polychrest. His idiosyncratic but wonderful turns of phrase are also present in numbers - "Omnibus routs me" - as he develops the humour that will shine throughout the series.
I recommend Post Captain as an ideal introduction to Patrick O'Brian's writings.