In brief, it concerns the events leading to the tragedy of an eminent judge forced to pass a death sentence on his own daughter by a secret first marriage when he was a young barrister.
All in all, it's a very readable novel, quite Hardyesque in its sometimes grotesque turns of fate, and it never goes entirely beyond the edge of believability. I am, however, not clear why Cecil's conflict of interest wouldn't be grounds for calling a mistrial. The scene of the judge dying after collapsing in court may well have been inspired by Thomas Noon Talfourd, an author and judge who died in 1854 after suffering an apoplectic fit while addressing the jury from the bench. The downside is that after reading more than a few Maxwell Gray novels, you start to spot leitmotifs such as the respectable central character tormented by a secret misdeed that comes back to bite him, and the feisty proto-feminist heroine with a trio of suitors (a Mary Sue if I ever saw one), and even locations (Mary Tuttiett had visited a few places such as Windermere before her lifelong incapacity, and her characters go there too). Unlike a number of her other novels, its setting isn't explicitly identifiable as the Isle of Wight, but the regional dialect and descriptions of a south-facing coastal location with a chalk down cut off by a sea cliff plausibly put "Swanbourne" as one of a number of manors somewhere slightly inland from Freshwater Bay or Alum Bay, either of which could be the coastal hamlet "Seagate" of which Cynthia thinks disparagingly:
... the little watering-place the sudden up-springing of which on her father's land was the chief source of her wealth. The sight of this, too, vexed her. Shortly before her father's death certain plots of his land had been sold on building leases. Hence a ghastly row of jerry-built stucco villas, a focus of cheap-trippers, bathing-machines, nigger bands and other horrors.(I suspect this is the author talking; a number of Maxwell Gray poems are similarly hostile to modernity, and she refers similarly to "the latter-day torment of mushroom villas" in another novel, The House of Hidden Treasure).
The Last Sentence is available on the Internet Archive (ID: lastsentence00hencgoog). It was filmed in 1917 (see the IMDb) with some wise changes in character names; the novel is overloaded with very similar C- names.
The Last Sentence is discussed in more detail in my book A Wren-like Note: the life and works of Maxwell Gray (Mary Gleed Tuttiett). See the official site, www.maxwellgray.co.uk