The beginning of the novel sets us up for what looks like it'll be a love story – we've got all the makings of a romantic comedy here. The prospect of a new life far from home is sad and scary to the Dashwood girls, but it prepares us (the readers) for adventures to come.
Sentimental Marianne has a field day over these idyllic, rather manic few chapters. Her immediate passion for Willoughby introduces us to the happy side of her emotional nature – when she's in love, she's truly unstoppable. Elinor is happy, too; things seem to be going well with Edward. Both sense and sensibility seem like they're working out… for the time being.
Marianne's unleashed feelings – the "Monster" here – take over. From this point out, her dramatically obsessive reactions take over pretty much everyone. Marianne's emotions are so strong that they don't stay bounded within her character; rather, Mrs. Dashwood and Elinor both get drawn into them. The dangers of overactive "sensibility" become evident. In addition, Elinor's own feelings get trod upon by Lucy Steele, whose announcement of her engagement to Edward stretches the limits of Elinor's practical exterior. Emotions are on the rampage all over here.
Marianne's fragile state is destroyed by Willoughby's cruel rejection, and the news of his engagement to Miss Grey. Her subsequent depression (and its result, her physical illness) demonstrate the worst dangers of sentimentality – Marianne allows her foolhardy feelings to overcome not only her intellect, but her body as well. Furthermore, Elinor's suffering is finally revealed to Marianne when Lucy and Edward's engagement becomes scandalous public news. It appears that nothing can possibly be resolved. This section ends with Marianne's life-threatening illness, the low point of the book.
On the evening of Marianne's miraculous recovery and "escape from death," Willoughby shows up desperately and spills his guts to Elinor. Our worries (and the Dashwoods') about Willoughby's feelings are finally resolved – we learn that he really does love Marianne, though he can't act on his feelings. Marianne herself gets better and resolves to do away with the "monster," sentimentality. She decides that she'll be more like Elinor in the future, and filter her passions through the screen of logic. Both of the love plots are resolved by the end, with Elinor and Edward married, and Marianne married off to Colonel Brandon, proof of the death of her personal scourge – her overactive romantic tendencies.