Tsukasa Domyoji, the heir to a vast fortune, announces his engagement to Tsukushi Makino, the ordinary girl who captured his heart, in a grand press conference that captures the world's attention. As the couple prepares for their wedding, they are summoned to a formal dinner with Tsukasa's powerful mother, Kaede. In a surprising turn, Kaede, who was once fiercely opposed to their relationship, warmly welcomes Tsukushi into the family and presents her with a priceless family heirloom: a magnificent diamond tiara known as the Smile of Venus, a legendary piece said to grant eternal love to its owner. The gift seems to signal a final blessing for the couple's future together.

Their happiness is shattered that very night when a mysterious thief crashes through the window of their hotel room, snatches the tiara, and vanishes. A frantic chase ensues, but the thief escapes. Stranger still, when Tsukasa and Tsukushi return to their room, the broken window has been completely repaired and the hotel staff claims nothing happened. Determined to recover the tiara and with their eternal happiness at stake, the couple embarks on a high-stakes investigation. Their search leads them from Tokyo to the dazzling casinos of Las Vegas. There, they gamble with millions of dollars and narrowly miss capturing the thief. Realizing they cannot succeed alone, Tsukasa calls upon his three closest friends, the remaining members of F4: the quiet and perceptive Rui Hanazawa, the playboy tea master Sojiro Nishikado, and the loyal Akira Mimasaka. Reunited, the group follows clues to a black market auction in Hong Kong, where they successfully win back the tiara.

Their victory is short-lived. Tsukushi becomes suspicious of Rui after seeing him with the thief, leading to a rift between her and Tsukasa. Before they can resolve their argument, they are drugged on a private jet, and the tiara is stolen once again. They awaken together, stranded on a remote, uninhabited island with no immediate hope of rescue. Forced to rely only on each other, the couple confronts the deepest questions about their relationship, their individual dreams, and the true meaning of a shared future. After they are rescued, they return to civilization to finally unmask the culprit.

In a final revelation, the thief is revealed to be an actor hired by their own parents. The entire international chase was not a criminal plot, but an elaborate, secret test orchestrated by their families, including Kaede Domyoji, to prove that Tsukasa and Tsukushi's love could withstand any obstacle, no matter how extreme. Having passed the ultimate trial, the couple returns to Tokyo, where they are married in a joyous ceremony surrounded by their friends and family. The story concludes with Tsukasa and Tsukushi back on the deserted island, now a place of fond memory, where they look to their future together, hinting at the start of a new chapter as they await the arrival of their first child.