16-year-old super-studier Nagi Umino, second-year student at the Meguro River Academy high school, was switched at birth. On his way to a dinner to meet his birth parents, he accidentally meets the brash, outspoken, Erika Amano, who is determined to make Nagi her fake boyfriend as she never wants to actually marry. But once Nagi makes it to dinner, he finds his parents have decided to resolve the hospital switch by conveniently having him marry the daughter his birth parents raised...who turns out to be none other than Erika herself! After Ai-chan's identity and love declaration become public, Nagi finds himself as an overnight celebrity. Meanwhile, Hiro, who's been a fan of Ai Mochizuki for a long time, is feeling very conflicted, and sparks are about to fly when she asks Nagi to meet him after school... To make matters worse, it's not long before Nagi and Erika find themselves locked in a hotel room to avoid the media! The fiery love pentagon gets even more intense!

16-year-old super-studier Nagi Umino, second-year student at the Meguro River Academy high school, was switched at birth. On his way to a dinner to meet his birth parents, he accidentally meets the brash, outspoken, Erika Amano, who is determined to make Nagi her fake boyfriend as she never wants to actually marry. But once Nagi makes it to dinner, he finds his parents have decided to resolve the hospital switch by conveniently having him marry the daughter his birth parents raised...who turns out to be none other than Erika herself! Nagi is feeling anguished after learning about Erika's older brother, so he goes to Erika's hotel mogul father to find out the truth. But his plan ends up backfiring as the situation grows even more complicated...though it's nothing a bike date with Hiro and a beach trip with all three girls can't fix! Time for Nagi's ultimate summer to begin!

After being swapped at birth, Nagi Umino is currently a second-year student at a prestigious high school. One day, he runs into Erika Amano, a spoiled rich high school girl who says she's on the way to meet the fiancé her parents arranged for her, and ends up pretending to be her boyfriend. But the two of them soon learn that they'd been swapped with each other as babies, and their parents decided to engage them to each other... Spoiled rich girl Erika Amano and son of restaurant owners Nagi Umino were swapped with each other at birth, and their parents have decided that the two of them should get married. On top of that, they've forced the couple to start living together! How will they cope with these cruel twists of fate?!

16-year-old super-studier Nagi Umino, second-year student at the Megurogawa Academy high school, was switched at birth. On his way to a dinner to meet his birth parents, he accidentally meets the brash, outspoken, Erika Amano, who is determined to make Nagi her fake boyfriend as she never wants to actually marry. But once Nagi makes it to dinner, he finds his parents have decided to resolve the hospital switch by conveniently having him marry the daughter his birth parents raised...who turns out to be none other than Erika herself!

After being swapped at birth, Nagi Umino is currently a second-year student at a prestigious high school. One day, he runs into Erika Amano, a spoiled rich high school girl who says she's on the way to meet the fiancé her parents arranged for her, and ends up pretending to be her boyfriend. But the two of them soon learn that they'd been swapped with each other as babies, and their parents decided to engage them to each other... Nagi and Erika have a new addition to their household: their younger sister Sachi! Nagi and Sachi have a big fight when he tries to send her back to their parents, but Sachi is actually just unable to say how she wants to spend more time with her big brother. Throw in an amusement park date with Nagi's beloved Hiro and some rapid bonding with Erika and this four-way love knot quickly grows more and more complicated!

At long last, Nagi and Hiro have finally started dating each other! Their first date resembles that first rainy day…but before long, Erika drags Nagi off to Okinawa?! Seeing Erika looking like a princess makes Nagi excited, and the arrival on the scene of Rihito, Erika's childhood friend and fellow hotel heir, makes him feel even more conflicted… And what's that about a proposal?! The storm of love has begun to rage in Okinawa!!

In the climax of the Okinawa arc, Erika and Hiro clash over their feelings for Nagi?! Meanwhile in Tokyo, while waiting for the others to return, Sachi has an unexpected visitor…from someone who had disappeared?! In the midst of all that, Nagi's mom Namie collapses… As he's on his way back to Umino-tei, Sachi hits him with the shocking assertion that he's “no longer her family”...