Meg Mackworth is a hand-lettering artist in Brooklyn, New York, known as "The Planner of Park Slope." She carries a secret: when stressed, she unconsciously hides coded messages in her lettering. Over a year before the novel begins, while designing materials for Reid Sutherland's wedding to Avery Coster, Meg sensed the couple was mismatched and embedded the word MISTAKE in the program by placing fairy characters on specific letters.
One Sunday, Reid walks into the Brooklyn stationery shop where Meg fills in for the owner, Cecelia. Meg is struggling with a creative block and a looming deadline: Make It Happyn, a craft-retail company, has invited her to submit planner treatments, and the winner will have a line mass-produced under their name. She is also lonely, as her best friend and roommate, Sibby, has grown emotionally distant.
Reid sets the wedding program on the counter and asks how Meg knew his marriage would fail. A quantitative analyst who works with mathematical patterns, he recognized the hidden code immediately. He explains that the wedding never took place and the breakup was mutual, and he promises never to reveal the secret. He mentions he hates New York and plans to leave by summer's end.
The next morning, Sibby announces she is moving in with her boyfriend, Elijah. Meg recalls moving to New York at nineteen after a devastating family conflict, arriving on Sibby's couch with nothing. She learned to love the city by studying its hand-lettered signs, which became the foundation of her career. Seeking to break her creative block, Meg finds Reid's business card and emails him, proposing they walk the city together to hunt for signs.
Reid agrees, but their first outing goes badly: signs are missing, it rains, and Reid interrogates Meg about her business finances. She walks away angry. To manage tightening finances, Meg accepts a new client referral: Lark Tannen-Fisher, a famous actress who wants a custom planner and two lettered walls in her townhouse. Lark's husband, Cameron, is a director whose controlling behavior unsettles Meg.
Reid sends a handwritten apology and a photograph of the sign they could not find. Meg calls him, and they try again. Reid suggests turning the sign-hunting into a game: Each person finds hand-lettered versions of the letters in the other's name. The playful format works. Meg sketches freely again, and they develop a routine of walks and letter games that deepens their friendship and unlocks her creativity.
During a Saturday walk, they sit in a park and talk for hours. Reid reveals he is one of seven siblings, started college at fifteen, and wanted to be a teacher but went into finance instead. Meg confesses her fear that Sibby no longer wants to be her friend. Reid tells her anyone would want to be her friend, "Especially me" (124). Meg recognizes her feelings are moving beyond friendship.
Tensions surface on multiple fronts. At Lark's house, Cameron belittles Lark in front of Meg, who speaks up. Lark asks Meg to leave. During a walk in Prospect Park, Reid presses Meg about what is bothering her, and when she deflects, he makes a cutting remark: "I suppose you could always hide it in some of your letters" (136). Meg retaliates, and she accidentally spills hot tea on his arm, revealing a patch of psoriasis, a stress-related skin condition he has been hiding. He leaves, and they do not speak for a week.
At a party, Meg confides in Lachelle, a calligrapher friend, who tells her that fighting is not always about leaving; sometimes it is about staying. That night, a bartender calls because Reid tried to pay his tab with Meg's business card. Meg finds him slightly drunk after a bad day. He confesses he watched all her tutorial videos to hear her voice and walked all day with a "Meg-shaped shadow" beside him (154). A brawl breaks out; Reid shields Meg and is injured. She takes him home.
The next morning, Meg lays out three points: She wants to be part of his life when things are hard; she needs assurance he has no grudge over the hidden letters and no lingering feelings for Avery; and New York is her home. Reid addresses each, explaining he and Avery were never right together. He cannot promise to stay yet. They share their first kiss and sleep together.
Over the following weeks, their relationship deepens. Meg completes one full Make It Happyn treatment and makes progress on Lark's planner. Reid works long hours on a project he will not discuss. Lark visits to apologize. When Sibby returns unexpectedly, the tension between the friends erupts. Sibby admits she has been jealous: New York was her dream, yet Meg became famous there. The fight escalates until Sibby makes a devastating reference to Meg's family secret. Meg tells Reid everything: At nineteen, she discovered that the woman who raised her was not her biological mother. Her father had had an affair, her adoptive mother had known, and they divorced once the truth came out.
Reid takes Meg to Maryland to meet his family. That night, he mentions the possibility of staying in New York to teach and accidentally refers to the city as home.
Back in New York, they run into Avery. Reid looks stricken, not from romantic feelings, but because he foresees how a coming scandal will affect her. Meg's pitch to Make It Happyn goes poorly, and she emerges to discover that the Federal Bureau of Investigation (FBI) has arrested Avery's father, Alistair Coster, who is also Reid's boss, for securities fraud. Reid is the whistleblower. A gossip blog publishes the wedding program with MISTAKE circled in red, linking Meg to the scandal. Reid's phone is confiscated, and he cannot reach her.
Sibby returns to help, fielding calls and managing the fallout. Meg asks whether their friendship only works when she is in crisis, and Sibby admits it might be true. Lark also comes, and the three women spend the weekend together.
On Sunday, an FBI agent delivers a handwritten letter from Reid. He explains that before the wedding, he discovered anomalies in Coster Capital's returns; the word MISTAKE reinforced his suspicions. He reported his findings and spent over eight months working secretly with authorities. He confesses he told Avery about the hidden message when they split, and her family's allies leaked it to the press. He declares: "I am sorry, and I love you, and the time I have spent with you has been the best time of my life" (279). The letter ends with GPS coordinates for every place they visited together, and one final set leading to a hotel in New Jersey.
Meg, Sibby, and Lark drive to the hotel. With help from the FBI agent, Meg reaches Reid's room. He is exhausted and guilt-ridden. She tells him she loves him. He says he loves her and is not leaving New York. Meg refuses Make It Happyn's revised offer to capitalize on the scandal, insisting she will rebuild on her own terms.
In the epilogue, roughly a year and a half later, Meg has launched Harbinger, her own line of journals and stationery. Lark, who has left Cameron and returned to acting, promotes the line on social media. Sibby and Meg have rebuilt their friendship, and Meg is designing Sibby and Elijah's wedding materials. Reid has left Wall Street, teaches calculus at a community college, and studies for his teaching certification. They live together in Meg's apartment. The novel closes with Reid decoding a playful hidden message Meg has left for him, their shared language of letters and numbers intact.