Tonight’s episode in this period series is called “Marked Twain” – and yes, indeed, it features a plot in which the esteemed author, in the latter years of his life, visited Canada to give a lecture, and was shot at by someone upset with his writing and views. Murdoch and company set out to protect this visiting American writer – and while the plot is intriguing on its own merits, what makes this episode worth a visit is the actor playing Mark Twain: It’s William Shatner. How does Shatner play Twain? You really must see it to believe it. And even then…