Yet another event is approaching in Sanditon, and once again a last-minute kink might derail it. Tom and Arthur are preparing for a ball that much of London’s high society is planning to attend, but the shopkeepers have refused to supply it until the army’s debts are paid. Tom comes clean to Arthur about his own debt to the army, and Arthur insists that he tell Mary while Arthur tries to salvage the ball.
But Mary is off to Lady Denham’s for whist. Lady Denham shares that she has heard from a friend that Colonel Lennox’s company has a reputation for drinking a town dry, running up debts, and then disappearing overnight. Since they are war heroes, there’s no legal recourse.
When Mary reports this troubling news to Tom, he in turn reveals his own debt, explaining that Lennox deliberately trapped him. She’s incensed: Have you learned nothing? she asks. After everything Sidney did for you? She must explain to her clueless husband that Sidney was in love with Charlotte; he married Eliza to settle Tom’s debts.
Charlotte is struggling to understand her own feelings. Georgiana and Alison keep encouraging her to return Lennox’s interest, but she is drawn to her employer Alexander Colbourne—although she has been avoiding him since his rude behavior at Lady Denham’s garden party. When they do finally cross paths, he tells her he regrets the way he and Charlotte parted then, but won’t apologize or explain.
After learning of Augusta’s fear that she won’t be able to attend the ball because of the garden party fiasco, Charlotte speaks to Alexander on her behalf. He offers a bit more of an apology, and then Charlotte argues that her position readying Augusta and Leonora for society is untenable if Alexander won’t let them attend events. Then you have some thinking to do, he responds.
Alison has decided to leave Sanditon without a thought after being disappointed by Captain Carter, but Charlotte encourages her to at least stay for the ball. Fraser has urged Carter to apologize to Alison—don’t force her to leave because of you, he says. But Carter, unlike Fraser, doesn’t even know the village to which Alison will return.
Fraser calls on Alison to ask forgiveness for his part in Carter’s deceit, explaining that he counseled Carter to tell the truth. He also reveals that Carter’s heroic war stories weren’t completely made up; they are his own exploits.
Georgiana continues to sit for Charles’ portrait of her, asking Arthur to be her chaperone even though he must soon leave. She and Charles convince Arthur that they will be fine on their own. Charles then tells Georgiana he is painting her for love, not money, and she kisses him.
Esther’s relationship is seemingly going less well. She is worried that she has not received any letters from her husband Lord Babington—thanks to Edward’s interference. She’s also behaving strangely—thanks to Edward’s replacement of the tincture prescribed by Dr. Fuchs for her inability to have a child with laudanum. Clara recognizes the symptoms of laudanum and accuses Edward of going too far. His plan is not just about protecting their son George, but also revenge on Esther. Edward insists that Esther attend the ball that evening, so that she can embarrass herself in front of Lady Denham.
Arthur has managed to save the ball by convincing the shopkeepers to wait a few more days for their debts to be paid. Tom tells him he had been looking for a replacement for Sidney and thus fell under Lennox’s sway. Arthur lauds Tom’s vision, promising him that he will figure out a way forward. Tom responds that he didn’t need to search for a replacement for his brother; he already has one in Arthur.
When Lennox asks Charlotte for her first dance, Alison immediately answers for her: yes. He’s delighted to hear that Charlotte is considering leaving the employ of Colbourne in the wake of the garden party, and tries to turn the conversation towards her future, but she quickly extricates herself.
Alison sees Carter regaling a gaggle of women and marches over to reproach him, praising Fraser in the process for coming to apologize. She turns down Carter’s galling offer to dance with her and warns him to be more truthful with other women. Fraser later asks Alison to dance, meeting her teasing remarks with compliments.
Georgiana and Charles dance, to Mary’s chagrin, but Arthur tells Mary that Charles can be trusted. Later, Charles and Georgiana talk privately and Charles reveals that he is leaving for Europe tomorrow, after he finishes her portrait. He asks her to join him, as his wife. Just then, Mary appears, so Georgiana cannot give an answer.
Edward tries to sow doubt in Lady Denham by mentioning that Esther’s family has a history of madness. Esther drops a glass proffered by Edward, who then gives her his own wine—laced with more laudanum. When she wanders onto the dance floor, Lady Denham grabs her, and Dr. Fuchs diagnoses her with hysteria. Lady Denham blames his tincture, but he reveals that it is just a placebo meant to give Esther hope.
Later, at the Denham manor, Lady Denham, Edward, and Clara are all startled to find George missing. Esther is holding him, comforting his crying—but the trio all fear for his safety. They have begun treating her like a madwoman.
Back at the ball, Charlotte is surprised to see Augusta and Alexander. Augusta is elated, telling Charlotte that she must always stay around, for the effect that she has on Alexander. When Alexander ends up dancing with Charlotte, he also asks her to stay: her presence has transformed them all, himself included.
Charlotte tells Alison she felt more alive than she has in a long time while dancing, but she’s confused why it’s with Alexander. Alison assumes she is talking about Lennox, who then appears and asks to continue his conversation with Charlotte. When he proposes, Charlotte refuses; she has been clear that she doesn’t want to marry anyone. He takes it badly and kisses her against her will, then yells at her when she pulls away.
Charlotte flees, distraught, and runs into Alexander, who spots Lennox and asks what he said. Charlotte angrily tells him what Lennox told her about Alexander, then chides Alexander for not explaining why he warned her against Lennox. He promises to tell her everything, but not at the ball.
He, Charlotte, and Augusta return to his home. Alexander explains that his wife Lucy stayed on in London for one more season when he inherited the home in Sanditon, then delayed her return and eventually stopped writing. Alexander went to the city and found that she was pregnant with Lennox’s child; Lennox had abandoned her.
Alexander was harsh with Lucy, and has always regretted it. After she gave birth to Lennox’s child—Leonora—she walked out into a rainy evening. She knew she was weak, and knew that she might become deathly ill. This is why it pains Alexander to be in Leo’s presence. Behind the door, Leo has been listening, and leaves.
Forgive yourself, Charlotte tells Alexander. Look towards the future, which could be a happy one. They kiss.