Hayate Ayasaki is an unlucky teenager who worked since childhood to make ends meet due to his parents' irresponsible behavior. One day he finds out that his parents ran away from home, leaving behind a massive gambling debt on his shoulders. While running away from the debt collectors, he ends up meeting Nagi Sanzen'in, a young girl who is the sole heir of the wealthy Sanzen'in estate, and due to a misunderstanding, Nagi ends up falling in love with Hayate. After he rescues Nagi from some kidnappers, she hires Hayate as her new butler. Aside from performing his ordinary duties as a butler, Hayate must fight to protect Nagi from harm, a difficult task as her life is always in danger because she is the target of other individuals coveting her family's fortune, and sometimes deal with some extravagant requests from her, oblivious to Nagi's true feelings for him.
Nagi is having trouble writing a new manga, so she orders Hayate to go out and gather references for her. She also gives Maria and Klaus the day off because she feels she needs to be alone to concentrate. Maria tags along with Hayate and the two soon become uncomfortable because their joint effort to gather references feels like a date. Yukiji, Risa, Miki, and Izumi spot them and try to get the two to kiss by bringing them to the trendiest date spots. At the end of the day, Nagi shows Hayate her fiery wrath when he shows her the photo references he took, which are mostly pictures of him and Maria having fun together.
On the way the school, Hayate and Nagi bump into Hinagiku who is worried about her sister after she gave her New Year's allowance. Yukiji appears, shocking the three with an unusually cheerful attitude, giving Hinagiku a New Year's allowance because of a promise Yukiji made with her 10 year ago (although it was only 500 yen), and shocking them even more after they found a letter of resignation which dropped from Yukiji's bag. When homeroom ends, the class begs Yukiji to stay as a teacher causing Shiori to burst out in tears, who is happy that the class cares for her and decides to not resign. The resignation letter belonged to Shiori who found being Yukiji's superior harder than she thought. Yukiji was happy because she found many signs of good fortune.
Nagi tries to gain experience after her work was rejected by a manga magazine. She believes she has no experience being lazy and staying in all the time, despite being rich and thus not having any problems living. She asks Hayate who has extraordinary experiences, but then becomes angry when Hayate tells her he once won a 'Manga Newbie Award'. She continues to gain experience while attempting to clean the house (which she messes up terribly) and by trying to serve food.
This episode, which is anime-only, takes place during the same time as episode eight and shows some other strange things that were going on in the background. A demon living inside an old video game is awoken and takes three of the student council members from Nagi's school inside the game where they must defeat her to return to the real world. Hinagiku, the student council president, arrives soon after and they go to destroy the demon, but she ends up ending it herself.
Wataru notices that Saki is acting strange so he decides to ask Hayate and Maria for help while he's picking up the cursed video that Nagi rented. It seems that Wataru forgot about Saki's seijin shiki and the people who watches the cursed video will be attacked by a giant cockroach. While Wataru is asking Nagi where he could buy a present, Hayate and Maria battle the giant cockroach. Nagi mistakes Wataru's present to be for Isumi so she sends Isumi to Wataru's house.
Klaus is feeling depressed after Nagi gives him permission to retire after he tells her about a dream he had, where he retired on a coffee plantation in Brazil. After Maria tries to convince Klaus to stay, Klaus decides to challenge Hayate to see who is the better butler. Klaus fails at this and has his mind set on retiring. Maria comes up with a plan that involves Hayate's female alter ego, Hermione. This plan fails as well and just before Klaus is about to leave, Nagi thinks of a way to make Klaus stay. Her plan succeeds and Klaus decides to stay as a Sanzen'in butler.
Nagi gets Hayate to first try on a female school uniform, much to his displeasure and before he can do anything about it, Maria comes to Nagi's room and sees what is going on. Nagi and Maria then proceed to put Hayate in a full cat-girl outfit which entices Tama's animal instincts. Afterwards, Hayate runs into Klaus and initially tries to run away, but in the end Klaus' instincts take heed, and Maria must interfere. Later, Hayate and Maria play a game of pool where if Hayate loses, his debt will increase to 200 million yen.
Hayate is searching for an ultimate attack and Maria suggests looking in the Sanzen'in library for hints. They find a book that teaches the Sanzen'in secret ultimate attack. After some training, Nagi finds out that Hayate is training for Hinagiku's sake and burns the book. Depressed, Nagi leaves the house at night where she meets Ayumu Nishizawa who challenges Nagi.
Kirika asks Hayate to wear the ancient butler robe which has been rejecting countless butlers. In exchange, she promises to leave Nagi and her fortune alone. Nagi, Hinagiku, and Himegami come to Hayate's rescue, but they arrive too late, as Hayate has turned into a nekomimi maid. They battle Kirika and Shion and during the battle, Nagi's pleas reach Hayate. The nekomimi maid uniform transforms into the legendary butler uniform. With his newfound power, Hayate defeats Kirika's snake, Jörmungandr.
Izumi, Miki, and Risa invite Hayate and Nagi to their "You-To-Be" Motion Picture Research school club where they film and gather funny movies and laugh at them. Nagi shows interest in the club and Hayate believes it's an opportunity to get Nagi to break her hikikomori habits. They are shown example videos which happen to be Hayate's embarrassing moments with voice overs. Hayate snaps and destroys the club's equipment and media just before Superintendent Kirika appears for her annual club investigation. It turns out the club is on the verge of being shut down and the trio were planning to use the video of Hayate as a means of keeping the club alive. As a backup plan, they promise Kirika new footage of Hayate jumping off the clock tower. With the club saved and Hayate and Nagi as new members, the club decides to hold a normal story competition. Risa tells the first story and soon the mic is passed around to even non-club members.
Hayate is running an errand for Nagi and meets Ayumu in the park. Still embarrassed about their last encounter, Hayate pretends he has amnesia and is unable to get out of the mess after Ayumu claims he is her butler. Nagi claims Hayate back and gets him to reenact experiences he faced in Nagi's mansion to recover his memory (even though he didn't lose them).
It's culture festival time and Hayate is kept busy by the students and teachers. Nagi searches for him, but she is unable to find him. A mysterious dark butler tricks Nagi and hypnotizes her. In her hypnotic state, she finds Hayate and leads him to the mysterious dark butler.
The superintendent of Hakuo Academy, decides to hold a Butler Battle Tournament to see who the strongest butler of all Hakuo is. At first, Hayate is reluctant to enter, but Nagi convinces him to do so so she can see him win. In the meantime, Hinagiku becomes a temporary butler so that she can enter the tournament to interfere with the superintendent's plans.
Nagi, still in her hypnotic state, helps organize the second day of the school festival with the help of Superintendent Kirika. They organize their own Comiket where Nagi sells her own dōjinshi. Kirika challenges Hinagiku to see if Nagi's dōjinshi or Hayate's butler cafe can make the most money. When Nagi tastes the cake that Hayate made for the competition, her hypnotic state goes away.
Sakuya visits Nagi. Nagi becomes offended when Sakuya tells her she's a brat because she has no siblings and claims she can be a better sister than Sakuya. Nagi orders Hayate to find kids that are lost on her property so she can prove it to Sakuya. Hayate comes back with Itsuki and Shunji, and Nagi plays with them while her SP looks for their parents. Sakuya suspects the kids to be thieves and catches them in the act. Afterwards, Kirika sends the third mysterious butler, Mini Yon, and he challenges Hayate to a Mini 4WD race.
Isumi is planning to come to Nagi's house but she ends up getting lost again. Fortunately for her, Sakuya finds her, but along the way, they find a man with the head of a carp who begs for their help. The carp man asks Isumi to phantom bust phantoms who are tampering with his lake.
Nagi completed an improved version of her manga, but still, no one can understand it except for Isumi. She takes Hayate, Maria, Isumi, and Sakuya out for ramen, but they find themselves in a hidden animation studio where they are shocked to hear that Nagi's manga is going to be animated. The directors declare Nagi's demands to be impossible, but they end up animating it anyways because of a rumor that their stock will go up if they do it. After the premiere one week later, everyone is at a loss for words.
Nagi surprises Maria with an overnight relaxation tour at the Hyatt Regency hotel after Maria becomes upset when Nagi and Hayate thought she looks older than her actual age. When Hayate orders for room service, they meet Izumi to find out the students of Hakuō Academy are running the hotel for a one day experience event. Maria enjoys a few of the hotel facilities until the students are unable to accomplish their jobs and ask Maria for help especially when the Kirika and Shion visits. Maria spends the rest of the day helping the students in secret from Nagi and Hayate.
Nagi is planning a celebration party for Hayate for graduating, though gets him to stay out of the mansion for a while by giving him the task of retrieving a small box carrying an item she bought off the Internet. While out, Hayate is offered 150,000,000 yen by a national escort service in order to recruit him into their business and so that he can pay off his debt to Nagi. While thinking this offer over, he loses the box and has to go halfway around the world in order to retrieve it, though by the time he gets back to the mansion its already dark out. Hayate is very grateful to Nagi for the party, and the item he retrieved which was intended for him, and turned out to be a small pocket watch. Hayate later turns down the offer from the escort service, and it turns out that Nagi's grandfather set up the offer in order to test Hayate's loyalty.
During Golden Week, Hayate, Nagi, and Maria take a trip to an island in the Aegean Sea. Nagi still does not like to go outside much, though she gives in to Hayate after he sulks. While out the next day, Nagi's hat is blown away by the wind, and when Hayate goes to retrieve it, he falls into a time slip and arrives many years previous when Nagi was living on the island. After he realizes what is going on, he discovers she is being targeted by the Mafia, and he ends up protecting her from them, ultimately winning; Hayate realizes that the reason he was sent back was to save Nagi. Hayate had promised Nagi in the past to watch the stars together at the beach, but when they go at sunset, Nagi's hat once again is blown away and Hayate goes through another time slip back to the present. This caused Nagi to remember Hayate in the past as a liar who disappeared on her and took her hat with him. Back in the present, Hayate gives Nagi back her hat and the two gaze at the starry night sky together.
Hayate defeats Klaus and Tama by catching Klaus' ribbon when falling off during Klaus' Ultimate Move, leaving the whole public under the impression that he has the power to defeat opponents without even touching them. After numerous retirements from Hayate's opponents and battles through the tournament, Hayate defeats Hinagiku in the finals and fights Shion. After defeating her by discovering her weak point, the Superintendent vows to bring Hayate to her by calling her Dark Butlers League.
With summer vacation coming to an end, Nagi is saddened that there won't be anymore free time to spend with Hayate. After reading a manga, Nagi decides to take Hayate fishing. While Hayate and Nagi are having a romantic time fishing (in Nagi's mind), the lord of the lake overturns their boat. Returning home, Hayate takes a bath and is joined by Tama who breaks the bath's water heater. Hayate becomes depressed because he believes he's always causing problems for Maria and attempts to fix the bath, only for it to be destroyed once again by Tama.
Hayate is having a bad day thanks to the pendant that Nagi's grandfather gave to him. After Hayate and Wataru rescue Isumi's stole, Isumi thanks Hayate by decursing the pendant. Nagi and Wataru become depressed because they misunderstand the decursing as an act of love because they embraced. Nagi runs away from home to Wataru's video store.
Nagi, perplexed by Hayate's apparent lack of weaknesses despite being a main character, plots ways to find chinks in his armor. As her various schemes fail to produce results, Maria, Sakuya, and Isumi get swept up in the flow and discuss their weaknesses as well. Hayate and Wataru begin studying German in preparation for a Hakuō mathematics exam, while Maria tells Hayate a bit of background information on Isumi, Sakuya, and Wataru. When they go to check on Nagi and Wataru, Nagi becomes jealous that they are alone together, prompting Maria to remark that Hayate's weakness seems to be not understanding the female heart.
Nagi and Ayumu compete together in the 4256th High School Girls Ultra Quiz tournament, though are each looking forward to the possibility of winning different prizes; Nagi wants the trophy for its resemblance to Hayate, and Ayumu wants the all-you-can-eat ticket to a restaurant so she can go out with Hayate. During the tournament, Nagi and Ayumu have trouble cooperating, though manage to end up winning the tournament in the end. Ayumu uses the all-you-can-eat ticket to invite Nagi, and Hayate to go out and eat together.