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! While out shopping with Erika one day, Hiro is suddenly and mysteriously abducted! Nagi feels he has no choice but to act, so he grabs Hiro's hand...and decides to elope?! To make things even more confusing, Nagi's long-lost childhood friend (and first love) Ai-chan is back from China, and she says she has an answer to what he asked her back then...

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!

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! The Megurogawa Academy culture festival has begun! Amidst their nervousness for the M-1 competition, Nagi and Erika walk around the school together. If they lose, their cohabitation will end... What's the “bomb” they've prepared to ensure their victory?! What is Soichiro planning?! And what's the answer that Nagi has reached?!

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”...