Thirty-year-old Mei Li Sirin Horwang has never had a boyfriend. She gets drunk at her best friend Ped Panissara Arayaskul's wedding and, driving home intoxicated, crashes her car. Her family scolds her and bans her from driving, forcing her to rely on public transport.
That night, Li accidentally catches their teenage maid Jarunee Boonsake having sex with her boyfriend on the rooftop. Li's father Jirawat Wachirasarunpat demands to see the boy's guardian, who turns out to be Loong Theeradej Wongpuapan, the man who helped Li the previous night. On another evening, Li encounters Loong again on the BTS Skytrain, where she accidentally breaks his sunglasses. She buys him a new pair and writes her phone number on the box, hoping he would call her. After waiting in vain, Li and her neighbor Plern Ungsumalynn Sirapatsakmetha meet Loong at the video rental shop he frequents, and the flirtatious Plern cleverly obtains Loong's phone number and learns that he is a night‑shift engineer for the BTS Skytrain. However, Plern refuses to give Li Loong's number, and instead becomes a cashier at the video rental store.
In retaliation, Li summons Plern's three boyfriends to the store, causing a ruckus that results in Loong's laptop being smashed. Feeling responsible, Li takes Loong's laptop to Ped's husband, but he can't fix it. Li goes to return it at the BTS office. When Loong finishes work, he finds Li, and they ride the BTS to get home. Loong decides to throw away the laptop and its bag. Li retrieves the bag and brings it home, where she discovers film negatives among the contents.
Li has the film printed and finds images of Loong with Kob Kavita Taksaorn Paksukcharern, an actress in "Saint's Tear." The photo printing shop owner posts the pictures online, drawing attention to their past relationship. Li later meets Loong again on a skytrain and explains that the pictures might have circulated because of her, but Loong clarifies that Kob Kavita is his ex-girlfriend and that their breakup was due to mismatched schedules.
The two then watch the stars at the Bangkok Planetarium and view an exhibition about an upcoming comet. Li asks Loong to watch the comet with her in Bangkok, and soon Loong invites her to celebrate Songkran by spending time together. During Songkran, Plern joins them, and Li finds herself uneasy about the festivities. Li learns Loong's address—the guesthouse by the Chao Phraya River—and she travels there, only to find Loong asleep; she eventually falls asleep beside him.
Before Loong leaves for Germany to study, he sends Li a box containing mementos: the mirror from her car, the damaged sunglasses, the broken laptop, Bangkok Planetarium tickets, and the memory card. Li studies the photos inside and hurries to Suvarnabhumi Airport to try to stop him, but she is too late. Loong watches the comet from his plane, while Li watches from the ground.
Two years later, Li unexpectedly reunites with Loong on a BTS Skytrain. He has returned to Thailand and works a day shift, and after they briefly reconnect at Siam Station, an extended train outage creates a moment of fate. As Loong calls her from the stalled car, Li realizes she has his number and, with the power restored, the two share a renewed sense of possibility for a future together.
