My First First Love opens with the 20th birthday party of YOON TAE-OH (Ji Soo). He’s in a fancy restaurant with his father, step-mom, and step-brother, lamenting the lack of gifts. Dad wants Tae-oh to be independent and buy him a studio apartment, but Tae-oh wants to move into his grandfather’s old house. Step-mom shoots down the suggestion and a bickering match ensues. He agrees to give Tae-oh the old house on the condition that he will drop by unannounced once a week to check if he’s bringing girls over, which of course, he never does.
Forward one year and Tae-oh is now 21, dancing around the house and wondering when he’ll have a girl to bring home. We jump another year to a 22-year-old Tae-oh telling his friend off for studying at his place. At 23, Tae-oh checks himself out in the mirror in anticipation of a blind date.
HAN SONG-YI (Jung Chae-yeon) wakes up in a house with ‘seized item’ stickers over everything. She uses bottled water to wash her hair and has unopened envelopes of eviction notices laying around. She calls her mom and says she’s been living in a house with no running water, asking her mom to pick up her calls.
Tae-oh comes over to give Song-yi a ride to college. He tells her this is the last time he drives her and she teases him, saying she plans to ride his scooter till she graduates. A flashback takes us to Song-yi’s father’s funeral in the same house. Tae-oh had wanted to be there for her, made her laugh and wiped her tears. He’d lost his mom when he was a child, and comforted her by sharing his pain of losing a parent.
Over time, the two had become close friends to the point that everyone at school thought they were together. Tae-oh had had a very obvious crush on Song-yi but once overheard her telling her friends that she’s never had any feelings for him. He’d tried to be annoyed by refusing to share his umbrella with her but she had pestered him and bickered with him adorably until Tae-oh cracked a smile.
Back in the present, Tae-oh reminds Song-yi that this will be her last ride with him since he’ll be dating a hot senior soon. She snorts and tells him hot girls don’t need to go on blind dates, and he teases her about never having had a boyfriend. Song-yi leaves to join her friends, who wonder how she doesn’t have feelings for Tae-oh despite being friends with him for 20 years. They inform her that Tae-oh’s nickname in his department is Mr. First Love because of his boy-next-door charms and easygoing nature.
There’s also a Mr. Last Love in the college—SEO DO-HYUN (Jung Jin-young), who comes across as intimidating and cold but has a dependable vibe that attracts girls. Turns out Mr. First Love and Mr. Last Love are fast friends despite being very different. They hang out at a cafe and Do-hyun talks about wanting to become a civil servant because of job stability while Tae-oh doesn’t believe in planning a lot and advises Do-hyun to be flexible.
Jeonghan University’s Professor Choi Seok-hwan attends his boss’ family event. When asked about his son, Professor Choi says CHOI HOON (Kang Tae-oh) is studying for college exams, only to turn around and see Hoon aggressively singing on the stage. Hoon gets punished for taking a gig at the said event. His dad kicks him out with nothing on his back, not even clothes.
A rich girl flanked by two attendants slips away to the washroom. In her bag is a backpack, a wig, change of clothes, and wads of cash. She disguises herself and manages to exit the airport without being noticed, leaving behind a note informing her mom that she’s off to enjoy a long overdue vacation at a friend’s house. The friend turns out to be none other than Tae-oh.
Song-yi receives a call at work and rushes back home to find a woman moving her stuff into her house. The woman tells Song-yi that she bought the house at an auction and gives her time to pack her belongings. Song-yi notices one of the staff—who turns out to be Do-hyun—packing her stuff and snatches it from him. Do-hyun notes markings on the door commemorating Song-yi’s elementary and middle school graduations and guesses that it used to be her house.
Hoon turns up at Tae-oh’s house wearing the dress he stole from his mom. Not finding his friend home, Hoon is left with no choice but to scale the wall and let himself in. The doorbell rings and he opens the door to find the airport runaway introduce herself as OH GA-RIN (Choi Ri), friend of Tae-oh. A teary-eyed Song-yi lugs her suitcase up to Tae-oh’s house, followed by Do-hyun, who has come to study. Meanwhile, Tae-oh is on his way to the blind date, unaware of the gathering at his house.