Hayate Ayasaki is a sixteen-year old boy who has been working hard earning money as a bicycle delivery boy. He loses his job after his employer finds out he is still in high school, and his irresponsible parents spend the money he had earned. Hayate tries to kidnap a young girl named Nagi Sanzenin, but fails in his attempt. Later, Hayate saves her from other kidnappers and Nagi repays him by offering him a job as her new butler. However, overcome by injuries received during the rescue, Hayate collapses and is taken from the scene by Nagi's private helicopter.

Awakening within Nagi's mansion, Hayate incorrectly concludes that he has died and gone to heaven, and resolves to make the most of it. While he is enjoying a soak in an enormous indoor bath, the water re-opens his wounds and he passes out. When he regains consciousness, Nagi and Maria explain that he is Nagi's new butler. Eager to demonstrate his aptitude for the job, Hayate begins cleaing the mansion at a remarkable rate, drawing on cleaning skills he gained during previous employment. He finds a manga that Nagi has been drawing, and when he mistakenly refers to it as a picture diary, Nagi ejects him from the room. Believing himself fired, Hayate leaves the mansion and is immediately abducted by the loan sharks that are after him. All seems lost until Nagi, wearing an unconvicing disguise, appears and rescues him by paying off his debt. Hayate will now work as Nagi's butler to repay her.

Hayate wakes up Nagi one morning to find a large white tiger sleeping with her in her bed. The tiger, named Tama, nabs Hayate and carries him outside to fight. Hayate inevitably loses. Afterwards, Klaus, who is still upset about Hayate being Nagi's butler, challenges Hayate to a match with a robot to see if he has the necessary skills to be a butler.

Hayate tries to motivate Nagi to go to school finally, and offers to take her to and from school everyday from now on. Nagi reluctantly agrees and goes to school for the day, but when she comes back it turns out she forgot her notebook at school. Hayate offers to go to school late at night to bring it back and at the school gate he runs into one of the school's teachers he met earlier.

After turning down Ayumu, Hayate continues to wonder what she is doing while at he is at the mansion. Transfer documents arrive in the mail for Hayate to transfer into Nagi's school, and Hayate is overcome with joy to the point of crying. Hayate attempts to study the rest of the day for the exam the next day, but continuously gets interrupted by Klaus and Tama, so he does not get much sleep. The following day, Yukiji Katsura, one of the teachers at Nagi's school, has a bit of fun at Hayate's expense in order to "loosen him up" before the real exam. However, due to the physical and mental exhaustion he acquires as a result, he fails the entrance exam by one point. Yukiji is told that she must go to the Sanzenin mansion to inform Hayate, but is hesitant to do so because she believes herself to be the reason Hayate failed.

Hayate and Nagi are excited about Hayate being qualified for Nagi's school. However, Yukiji Katsura struggles to tell Hayate that he actually failed and that she was the indirect cause, having played pranks on him before the exam. She continually gets beaten by her little sister for skirting around the issue. Hayate eventually finds out and is disappointed but finds out that he actually qualified thanks to a recommendation from Maria.

Hayate goes to school early with Nagi despite her complaints. They meet a butler with a student along the way who eventually runs off with Hayate's necktie which makes Hayate and Nagi fetch a new one from home. Hayate has various discussions with Nagi and discovers that she jumped a few grades and thus is a high schooler despite her age. Hayate also discovers that Nagi used to be in a kendo club with Hinagiku but quit. Hayate decides to put Nagi back into the club when he is challenged by someone in the club.

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.

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.

Nagi decides to take Hayate to an amusement park in her own backyard. While Hayate is roaming through the haunted house, Robot Eight, who was defeated in episode 3, appears to get revenge. After Eight's plan fails, he flees, upgrades, and breaks into Nagi's mansion for another chance for revenge.

Shiori Makimura is sent to take over Yukiji Katsura's homeroom at Hakuō Academy after a number of failed projects at M.H.E. Corp. Yukiji Katsura is demoted to sub-homeroom teacher. In order to get her old job back, Yukiji begs Hayate to help her blackmail Shiori. Hayate refuses, but Shiori runs away crying and calls Eight after Yukiji manages to get a picture of Hayate on top of Shiori after he falls down.

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 is despondent over her conflict with Hayate, so she, Saki and Gilbert plan a fake kidnapping in Isumi's house underground. However, things turn bad when a Mecha programmed by Shiori goes crazy, and it is up to Hayate to go rescue Nagi and to recover his butler's status.

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.

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.

Yukiji is found murdered at a hot spring inn and Detective Nagi decides to take it upon herself to solve the mystery. After a number of wrong assumptions, Nagi blames the voice in the sky ("Ten no Koe", i.e. the narrator). Kirika sends another butler to defeat Hayate, and they have a B-Daman battle.

When Nagi goes to school, she forgets to bring her lunch and Hayate offers to bring it to her. Once he gets to school, he is stopped at the front gates by a teacher (Yukiji) who thinks he is an intruder. Hayate eventually gets into the school, but the teacher continues her pursuit. Once inside, Hayate meets the student council president of the school, Hinagiku Katsura, for the first time, and helps her down from the tree she was climbing. As a reward, she brings him to the top of the Student Council tower so that he could see from the terrace. Yukiji, who is still searching for Hayate, finally finds him there and ends up fighting Hinagiku. After taking Nagi's bento box from Hayate, she jumps onto the railing of the terrace only to get blown off by a breeze. Hayate promptly rescues her, which ended her hostility to him. In the process, however, she had thrown Nagi's bento box onto to ground and spilled its contents.

Nagi does not want to go to school today and instead stays home to hang out with some friends. One of her friends, Sakuya Aizawa, arrives and starts to teach Hayate to be a better comedian despite Nagi insisting that he is a butler. Hayate later goes out to pick up some more tea wearing a cashmere coat, but finds the journey perilous.

Continuing from the last episode, Isumi gets to Nagi's mansion but Hayate leaves to pick up the tea that he was supposed to get. Hayate is ambushed by Isumi's protective staff and the loan sharks from the first two episodes. Hayate eventually makes it back, but comes down with a cold. The next day after Hayate has healed, he meets a young boy and his maid who have come to Nagi's mansion. The boy, Wataru Tachibana, is Nagi's fiancé.

Wataru, who is infatuated with Isumi, challenges Hayate, whom he perceives as a rival, to a duel. Hayate loses to Wataru on purpose, hoping for a romantic conclusion between Wataru and Isumi that will allow Nagi to break off her unwanted engagement. However, his overly obvious act ends up hurting Wataru's feelings. In order to reach Wataru and apologize, Hayate, together with Isumi and Nagi, must brave the Tokyo subway system.

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 ready to fight. However, the boy who challenged him calls his butler to fight instead. The butler then punished the challenger for being a coward. Hinagiku then stops them and the battle starts. Hayate stands no chance against the butler since he lacks a last-minute powerful move. However, he eventually wins by using the boy who challenged him as shield calling it the "Art of Changing Bodies."

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.

The students at Hakuō Academy go to a remote island for beach class. The butler of the beach resort named Schmidt hen Bach greets them and tries to create fun experiences for the group. However, the group of students is not having fun and want to go home, but Schmidt is not willing to let them go.

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.