Marianne wonders about the nature of Elinor's affection for Edward, and takes it upon herself to find out. She just can't understand her practical, cautious sister.
Upon leaving Norland, Marianne is inconsolably sad.
At Barton, Marianne takes an instant disliking to all of the Dashwoods' new neighbors and friends – though she admits that Colonel Brandon's attitude towards music deserves some respect.
Marianne dismisses the Colonel in an argument with Elinor, saying that he's so old, he's practically dying (the poor guy's only actually 35).
Marianne plans to take walks around the beautiful countryside to pass the time. One day, she and Margaret go out on such a jaunt. They're caught in the rain, and take advantage of the excuse to run in an unladylike fashion on the way home.
Running down the hill, Marianne trips and sprains her ankle. She's assisted back home by a mysterious gentleman who's out hunting.
The young man's name is Willoughby, and Marianne is overwhelmed by his courtesy, handsomeness, and general romantic nature.
Marianne falls head-over-heels for Willoughby in a matter of minutes.
Over the next few days, Marianne grills Willoughby on a number of important topics (like poetry and music), and decides that they're meant for each other. They are inseparable, and together they make fun of poor Colonel Brandon, who's still clearly smitten with Marianne.
Marianne accepts the offer of a horse from Willoughby, but Elinor makes her refuse.
Marianne gives a lock of her hair to Willoughby – a very significant gift. Everyone's sure they're engaged.
On the day of the proposed excursion to Whitwell, Colonel Brandon has to leave, so plans change – Marianne and Willoughby go for a private drive in his carriage, and end up touring his aunt's home at Allenham. Marianne tells Elinor all about it, despite the latter's horror at her sister's unthinking forwardness.
Willoughby is engaged to come and have dinner with the Dashwoods, but instead, he shows up early and tells Marianne that he's got to leave Devonshire to go to town.
Marianne is completely overwhelmed by sorrow and self-pity. She does nothing but cry and swan around, thinking of her good times with Willoughby.
Marianne passes the time in his absence by reliving their favorite moments, reading their favorite books, and stormily playing the piano and singing. She's obviously wallowing in her pain.
One day, a gentleman rides up on a horse – Marianne is sure that it's Willoughby, but it's actually Edward.
Marianne describes her perfect income and life to Edward and Elinor – basically, it's what her life would look like if she were married to Willoughby.
Upon the arrivals of the Palmers, then the Steeles, Marianne makes her dislike of both parties fairly evident. Since Edward's gone, she's returned to her habit of sulking and thinking about Willoughby all day.
Mrs. Jennings invites the Dashwood girls to London for the winter. Marianne, knowing that Willoughby's there, is all for it. She's uncharacteristically gracious to Mrs. Jennings. Elinor grudgingly agrees to go after their mom gives her approval, and they're all set.
As soon as they arrive in London, Marianne starts sending letters to Willoughby – but no reply comes. One day, they return home to find his card there, and Marianne insists upon staying home every morning in case he visits again.
At a fashionable party, Marianne sees Willoughby and confronts him. He's there with another girl, and greets her coldly. Marianne has a fit, and the Dashwoods have to leave.
Marianne writes Willoughby another desperate letter, and finally receives one in return, which basically tells her that he doesn't care for her, and never has. He returns her lock of hair and all of her other letters. Marianne admits that she and Willoughby were never actually engaged.
Marianne falls into a despondent slump – without Willoughby, nothing seems to matter to her anymore.
Elinor and Mrs. Jennings try to coax Marianne out of her melancholy, with little to no result. She just wants to go home to Barton to be with her mother.
Marianne cheers up briefly when Edward's around, thinking that things are still OK with him and Elinor, at least (little does she know that they're really not).
Marianne finds out about Lucy and Edward's engagement along with everyone else, and feels terrible that she's neglected Elinor's feelings for so long – she'd thought Elinor was just heartless, and now feels just awful about her sister's heartache.
Marianne's wish to go home is granted, and the Dashwoods are set to leave with the Palmers and Mrs. Jennings; they'll stop at Cleveland, then proceed home to Barton.
While at Cleveland, Marianne takes long, melancholic walks in the rain, thinking about how close Willoughby's home, Combe Magna, is.
As a natural consequence of her long walks, Marianne gets sick – then grows deathly ill. Everyone thinks she's going to die.
Marianne pulls through in the end – everyone is tremendously relieved, including Willoughby himself.
Marianne grows stronger and stronger, and the Dashwoods go home to Barton.
It's obvious that something has changed in Marianne; she's realized that her foolishly sentimental behavior has been the root of most of the problems in the book. She vows to have more sense in the future…she only wishes that she could know what Willoughby thought of her.
Well, she can. Elinor reveals Willoughby's story to her sister – he still loved her after all, but had to dump her for his fortune. Marianne is struck with grief, but seems to be OK (relatively).
Elinor accepts Edward's proposal, and Marianne actually falls into the background for the first time. She starts living more quietly, and eventually learns to return Colonel Brandon's affections.