Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Melancholic
Apart from “Gareth and Lynette,” which is probably the most lighthearted of all of the Idylls, the tone of Idylls of the King is one big bummer: melancholic, mournful, sad, and even downright depressing at times. That’s probably because this story is all about how and why the fellowship of the Round Table failed due to the flawed nature of human beings. Yeah, that's a bummer right there.
Even seemingly happy moments can’t entirely escape the shadow of the Round Table’s coming collapse. When Lancelot and Arthur pledge their undying love and loyalty to one another, for example, it should be a happy occasion: “And Arthur said, ‘Man’s word is God in man; / Let chance what will, I trust thee to the death” (“Coming,” 132-133). But Tennyson’s audience would have known that Lancelot would go on to betray Arthur in the worst possible way, by sleeping with his wife, which makes this moment sad and a little bit ironic.
Another moment that pretty much sums up the tone occurs at the end of “The Last Tournament,” when, “in a death-dumb autumn-dripping gloom” (750), Arthur stumbles upon his court jester, Dagonet, in front of Guinevere’s bedchamber. When Arthur asks Dagonet who he is, his response—that he’s Arthur’s fool who “shall never make thee smile again” (756)—kind of sucks all the joy out of things for a while. The Idylls of the King are filled with moments like these, which make a sad story that much sadder.