The title also happens to be the plot outline. Elements of the plot have been duplicated in countless books, TV shows and movies. Army of Darkness and MacGyver leap immediately to mind. The book is a fantasy, and if haters can set aside its numerous anachronisms (A man from 1900, for example, would never be able to understand the language of 6th century England), it's quite enjoyable.
The novel is considerably more adversarial than one might expect. The main character is uncouth, obnoxious, and a jerk, even more so than is necessary given the immensely frustrating ignorance of the 6th century people. I suspect Twain plugged himself in to the Boss character, and had a good old time writing this one.
The main character is out to get the established Church, not in a no-holds-barred, Philip Pullman way, but in a logical way that recognizes the value of faith while tearing down the humanistic and suppressive political and economic machinations of the Church.
Twain also takes shots at England through the ages, at its historically oppressive caste system and at the English people's long-running love of hereditary nobility.
Commentary on politics and on human nature abound, but A Connecticut Yankee in King Arthur's Court is still a great adventure story. These two elements step on each other's toes sometimes, but Twain pulls it off.
Clunky title. Great book.