When the novel opens, Connie and Clifford have just made it back to Wragby after Clifford's recovery from his World-War-I-inflicted wound. Wragby is ugly; its coal-mine is worse; and Clifford is impotent, and also kind of a jerk. Welcome home, Connie.
Life at Wragby isn't much of a life. It's more of a half-life, or a non-life, or, well, as Lawrence puts it, "a life: in the void. For the rest it was non-existence" (2.35). The people of the Tevershall village don't like Connie; Clifford's friends patronize her; and Clifford himself sees her as a glorified nurse. Both Connie's dad and her sister Hilda worry about her, but they come up with different solutions: Hilda takes her to London to perk her up, while her dad thinks she should take a lover. Mostly, she spends a lot of time looking at flowers and thinking about how much her life stinks.
Eventually Connie follows her dad's advice and picks up a boyfriend, the Irish playwright Michaelis. Like most new relationships, it's great for a while: lots of sex, no strings attached. And then it ends in flames, when Connie rejects Michaelis's marriage proposal. He mutters some nasty stuff about frigid women, and Connie swears off men. Like a Virgin, Pt. 2.
Let's let D. H. speak for himself:
And then began again the unspeakable motion that was not really motion, but pure deepening whirlpools of sensation swirling deeper and deeper through all her tissue and consciousness, till she was one perfect concentric fluid of feeling, and she lay there crying unconscious inarticulate cries […] The man heard it beneath him with a kind of awe, as his life sprang out into her. (10.305)
The climax of the book is, literally, a climax—and not just any climax, but a simultaneous one, which Mellors practically gloats about. We get it, Mellors: you're good in bed. Congratulations; have a cookie, and please wear a condom.
Anyway, there's no going back now for Connie. She definitely can't return to Clifford, even if it means giving up Wragby and her title and being plain old "Mrs. Oliver Mellors."
Sex is awesome, and Connie has it at every opportunity: in the forest, in a rainstorm, in the hut, in Mellors's cottage. She even sneaks out like a teenager to spend the night with her boyfriend. Obviously this can't last. We're not in a Harlequin Romance here; this is Serious Literature, so someone's going to get shamed.
While Connie's off in Venice trying to convince her family to help her get a divorce, Mellors's drunken wife shows up (naked in his bed, to be precise) and causes trouble. She figures out that Mellors's lover is Connie and starts spreading it around the neighborhood. It's ugly, and it gets uglier. When Connie returns, she tells Clifford that the story is true, and he says some truly awful things about animals and the lower classes.
At the end of the novel, Mellors and Connie are living happily in a cottage in the north of England, with their adorable little baby. Except not. Connie is still trapped at Wragby and Mellors is stuck learning how to milk cows while they wait for Mellors's divorce. Happy endings are so last century.