A new semester is about to begin, but Nagi is lazing around as usual. Hayate and Maria try to cheer her up without success until she gets impressed by a TV Show about aliens near Area 51 in Nevada, USA and wishes to go there to investigate. An opportunity arrives when she receives a call from the U.S. Police asking her to retrieve some of her late father's belongings, but Maria states that she is forbidden to go there until Silver Week. Nagi flees from the mansion in anger just to find later that she had forgotten her cellphone and wallet and is unable to get back home. However she is found by Hayate and when he is approached by a small child, he finds that Nagi was kidnapped. Hayate gets on the girl's bike to pursue the kidnappers and rescues Nagi from them. Just then Hayate and Nagi return home, the girl appears before them once more, introducing herself as Ruri Tsugumi and claiming to be Nagi's little sister.

Despite her shocking revelation, Ruri is ignored by Hayate and the others until she is finally brought before Nagi who hears from her that she is actually her sister, despite having no proof of it. Nagi allows her stay for a while, but after seeing her snooping around the mansion, she decides to take her to live with the other tenants of the Violet Mansion. Ruri refuses to stay, until Nagi is convinced to give her a chance. Nagi asks Ruri if she has any sort of special skill and she reveals herself as a master of Systema, strong enough to confront Hayate. Hayate is attacked mercilessly by her until he decides to fight back and she starts crying, which is later revealed as just a trick for him to open his guard. Nagi reprimands Ruri for using such underhanded tactics but compliments her for her fighting skill, even asking to learn it from her.

At Hakuo Academy, Ms. Katsura tells Hayate to deliver student council notes to Hinagiku. Hayate finds Hinagiku napping when Miki, Izumi, and Risa appears, waking her up, judging that Hinagiku is often scary. Hinagiku decides to be nicer for Hayate. Despite hearing her favorite tea cup get broken, and her sister already spend the money that she lend to her, Hinagiku shows a smiley personality to them; much to their surprise. Hayate suddenly ask Hinagiku to see a kids’ movie with him then Ruri joins them later. The kids in the theater tease them thinking that they are a couple, the movie starts shortly after but the film cuts off. Then Hayate takes Hinagiku to an amusement park; leaving Ruri behind. They enjoyed themselves until it was time for them to leave. Hinagiku ask Hayate to why did he take her out, he explains it was Miki and the others who told him to; thought she was having a bad day. Hinagiku says that she wouldn’t go out with a guy unless she likes him, but Hayate couldn’t hear that part.

Ruri is looking for a way to get close to Nagi in her search for some unknown item at her mansion. Chiharu and Kayura recommend her to buy some games for her to play with Nagi just to find later that she had already bought and played them. After both Nagi, Chiharu and Kayura have a heated discussions about their preferences related to manga, anime and games, Ruri asks them for some recommendations and they set for Tachibana's shop. While the trio argue about which manga she should start reading first, she ends up wandering into the R18 dōjinshi section just to flee in horror soon after. In the end, she ends up purchasing just a dōjinshi from an unknown author which Hayate recognizes as one of Nagi's original creations.

Angry at Maria for always scolding her, Nagi orders Hayate to follow her arround looking for some reason for her to scold the maid herself. Unable to find any bad trait about the maid, the butler stumbles on Yukiji who invites him for lunch just to learn that she has no money and intends to have him pay the bill. Meanwhile, Maria calls Mikado and asks about why there is no pictures or any info about Nagi's father, and he responds that all evidence of his existance was erased by him, as according to him, he was just a con artist who married Yukariko aiming for the Sanzenin fortune. As Hayate argues with Yukiji, both Ruri and Sonia appear and start ordering food, increasing the bill. As the four argue about who will pay the bill, a thief breaks into the restaurant, giving an opening for both the sister and Ruri to flee. The thief claims that he is stealing money for the sake of his sick little sister's medical expenses and Yukiji reprimands him while pointing his own gun at him and Hayate. Hinagiku appears mistaking her sister for the thief and stops her. With the thief turning himself to the police and Hinagiku leaving with her sister, Hayate remains alone at the restaurant, calling Maria to ask her for help with the expenses.

Silver Week has arrived but Nagi has no interest to set to Las Vegas and retrieve her late father's possessions, rathering to spend the day with Hayate instead. Later she pays a visit to the Violet Mansion, just to find that Ruri is alone there. Unable to leave when it starts raining, Nagi manages to have Ruri confess that her true intention is to obtain the elusive "Black Camellia", a treasure believed to be in possession of the Sanzenin family and Nagi asks her grandfather by phone about it. However, Mikado refuses to say anything beyond the fact that the treasure was stolen 13 years ago and she breaks her smartphone in anger. When the rain worsens into a storm, the two girls struggle to keep the house by themselves when Ruri saves Nagi from being hit by a large tree branch carried by the wind. When the storm passes, Hayate appears at the mansion and he gets a call from Mikado who reveals to Nagi that the Black Camellia was stolen by her father.

A story about Nagi’s father, Shin Hayek, is told how he was a thief then he met Yukariko and eventually fallen in love with her. Nagi discover the Black Camellia is in Las Vegas with Shin’s belongings. Nagi explains the Black Camellia is a clock that brings bad luck (but can grant the owner miracles for eternity). Ruri who is desperate decide to enter a quiz contest hosted by Fumi Hibino to win tickets to New York as a way to go to Las Vegas. In the end, Hinagiku and Ayumu won and want to see Ruka’s concert in Las Vegas. Ruri ran away wondering what to do next, since Nagi warned her not to do anything dangerous. Hayate comes looking for her when he, Isumi, and Sakuya witness her being abducted by a bird-like creature.

Hayate, Isumi, and Hinagiku (using her Shirosakura sword) manage to save Ruri from the bird-guy, but she refuse to tell them who he was. The next morning, at the mansion they receive a ransom note for Ruri’s return in Las Vegas; nonetheless, recognize the handwriting to be Ruri’s. Wondering if she really was kidnapped or not, Nagi finally takes the motive to go to Las Vegas and retrieve the Black Camellia. Testing the misfortune clock, Maria grabs it then suddenly the ventilation went up under her skirt in an eerie timing, and so Hayate took possession of the clock for now. While killing time in Las Vegas, Hayate got lost and his smartphone battery unfortunately died. He unexpectedly meets Ayumu saying their plane had a mechanical problem, thinking the Black Camellia caused it. Maria and Nagi found Hayate, from their viewpoint seeing as if he is kissing Ayumu. Much to Nagi’s shocking surprise just to find out he was only taking a bit of fuzz off Ayumu’s eye.

While entrusted with the Black Camellia, Hayate's bad luck has gotten much worse. At the hotel they’re staying, the trio: Izumi, Miki, and Risa overreacts to help search for Ruri, as Hayate tries to cease them two cops mistake his action as of indecency, then startup panic throughout the hotel. Nagi insist Hayate to leave when Yukiji takes the clock from him, she won billions from a casino but then thieves showed up and stole the money and the clock with her car. Later, Hinagiku receive a call from Izumi to where the thieves' hideout. Nagi goes along with Isumi and Sakuya to where Ruri is supposed to be at. There they find a tree in the desert and meets a woman who appears to be a demon, when Nagi tells her the Black Camellia been stolen the woman sends out Ruri and the bird-guy—Shidou to retrieve it while taking Nagi hostage. Hayate and Hinagiku saved Izumi and Yukiji from the thieves before Ruri appears and found the blond-hair guy who has the clock. Ruri knocks out the thieves using her skills, but just as she strikes the guy with the Black Camellia it suddenly transforms into a sword, Hayate pushes her out of the way and gets stabbed.

Nagi inquires the woman why does she desire the Black Camellia, saying it is a magical sword that grants eternity. When Hayate got stabbed by it he collapse and later gets up, however his personality seems to have changed. The woman and the tree appear and attempt to take the Black Camellia, the thieves try to fight back as their hideout catches on fire and escapes with Yukiji’s money. Hayate wants to talk to the woman, but Isumi interrupts them and release Nagi. The woman starts to weaken and retreats with Shidou and Ruri. As the police arrive, Nagi concerned about Hayate’s behavior, and then Hayate tells her that he "quits" being her butler, leaving Nagi alone. The next day, Nagi pretends to be okay without Hayate. They meet Kyonosuke with a new car saying Yukiji will pay him back for it wasn't aware that she was robbed. Kyonosuke takes Nagi and Ayumu a drive to Area 51 when Ayumu shows Nagi the Black Camellia she found last night. Then the car suddenly broke down, Kyonosuke and Ayumu left Nagi alone in the desert. Nagi begins to realize how important Hayate was to her and exclaim that she wants him back no matter what. Ayumu comes back and gives Nagi a helping hand. Meanwhile, Hayate minds his own business when Yukiji gets him to find her money by interrogating a thief that she caught. Hayate (now speaking English) reluctantly threatens the thief who gives them the location at the Starside Hotel.

Everyone gather together gladly to help Nagi search for Hayate. The girls try to figure out the mystery: the Black Camellia, the woman—Dolly, the accident 13 years ago, and Hayate’s strange behavior. After Izumi and Hinagiku mention that Hayate got stabbed the previous night, Nagi comes to a conclusion. Ruri appears and Nagi asks her to bring her Dolly’s ring; Hayate’s life depends on it. Nagi orders Klaus to deliver her mother's wedding ring over immediately then lends it to her friend, Ruka Suirenji, so Hayate will notice the ring on Ruka’s TV interview. Ruri found Dolly's ring and receive an email to meet at the Starside Hotel. Tonight at Ruka’s concert, Nagi’s friends go there to enjoy the show. Hayate and Yukiji arrive at the hotel; Hayate goes into Ruka’s dressing room and found a note written by Nagi informing him the ring is at the wedding hall. In the wedding hall, Nagi reveals the Black Camellia is a sword that exchanges a person’s soul and what happened 13 years ago, and thus Hayate is now Shin Hayek—Nagi’s father! Shin sat down with Nagi and talk about Yukariko. Dolly shows up with a captured Ruri and demands the Black Camellia, but Nagi and Shin explain that her beloved king died 13 years ago and Hayate is inside the clock now. Shin demands his ring back that signifies his and Yukariko’s eternal love. While Yukiji beats up the thieves, chaos rises above her and Maria cause by Dolly.

Within the Black Camellia, Hayate is introduced by a tanuki who tells him how the clock’s fortune functions. Meanwhile, Ruka is doing her singing performance; Isumi and Sakuya run to help Nagi. Ruri and Shidou try to defend Dolly but she rejects them, Shin protects Nagi from Dolly’s wrath. The rest of the thieves are defeated when Hinagiku uses her Shirosakura sword to rescue Ruri from a falling pillar. Shin takes Nagi to safety and approaches a helicopter with Hinagiku and Yukiji (with her money), but then Dolly captures Nagi and creates a gigantic tree. Yukiji flies away with Hinagiku on a parachute, Dolly begins to crush to Nagi, and then Shin arrives with the Black Camellia and stabbed himself asking Nagi to get his ring back for him. Shin appears before Hayate telling him to go save his daughter. Hayate comes after Nagi, proving to Dolly that he is not her beloved and starts to crumble. Hayate holds Nagi’s hand and flies down on big balloons miraculously and she confesses to him. It’s raining money, Isumi and Sakuya had Ruri taken to a hospital. Shin sang a song when Yukariko appears and the two are finally reunited. As Hayate and Nagi land safely Dolly comes down and Nagi reveals what her beloved king last words were before disappearing into the sky, the Black Camellia got crushed. And they go to see Ruka’s concert. After all other businesses were taken care of; Ruri gives Nagi her father’s wedding ring and leaves with Shidou revealed be a handsome man in a suit. Hayate and Nagi arrive at Shimoda to drop off Shin’s ring and the Black Camellia at Yukariko’s grave thanking them for everything.