The novel begins as Clarissa prepares for the party she’ll give that evening. First stop: a trip to the florist. It’s a big deal that Clarissa is doing some of the work of putting the party together rather than just planning it. Her parties mean a lot to her and she puts her heart and soul into making them perfect.
Clarissa’s old suitor, Peter Walsh, drops in on Clarissa unannounced. They have a short visit in which it becomes clear that he’s nowhere near being over his love for her. After a humiliating sob fest (on Peter's part), Clarissa invites him to her party as he races out the door. Now that Peter's back, will Clarissa start to doubt her relationship with her husband? Will she pine for the past? All we know is that ex-boyfriends are never good news.
We now meet Septimus, who’s waiting for an appointment with the eminent psychiatrist Sir William Bradshaw. Septimus' presence isn't a complication to the main conflict (yet): he doesn't really even cross paths with any of the other main characters. That said, his trauma-induced anxiety complicates the simple view that the English people want to have about the war. War is not all about heroism, and Septimus is a strong reminder of the scars that it has left on society. This isn't your regular plot-driving complication, but a deeper, more meaningful one.
After a moment of joy with his wife, Lucrezia, Septimus decides that he won't go with the doctors to a mental institution. Instead, he throws himself out a window and is impaled on the railings below. The contrast between the joy of the couple's conversation and Septimus' suicide makes this scene even more climactic.
Woolf leaves the reader in the dark about the relevance of Septimus and his suicide to the rest of the story. What could a shell-shocked World War I soldier have to do with a fifty-two-year-old society lady? The narrative jump back to Peter's thoughts and Clarissa's party forces us to wait even longer for an answer.
Dr Bradshaw and his wife arrive late to the party and Lady Bradshaw excuses them by explaining that one of her husband's patients had committed suicide. Clarissa is outraged that anyone would mention death at her festive gathering, and she retreats to her bedroom to collect herself. (We're starting to see how it's all connected now.)
After a few moments of reflection, Clarissa is no longer offended by Septimus' suicide, but rather identifies with it. She feels that he’s made a beautiful and sublime sacrifice that allows her to see life with fresh eyes. She returns to the party a somewhat different person, and her joy spreads immediately to Peter Walsh.