Three siblings, Shōma, Kanba and Himari Takakura are living together, despite the fact Himari was given only a few months to live by the doctor. The three visit an aquarium to see the penguins, where Shōma buys her a queen penguin hat. However, Himari collapses and later dies in hospital. As the brothers mourn, they are surprised to see Himari suddenly spring back to life and cured of her condition, possessed by a spirit which has decided to extend her life, though she returns to normal when her penguin hat falls off. The next day, the siblings receive a shipment of frozen things, which are later revealed to be a trio of strange penguins that no one besides them can see. Just then, the hat possesses Himari again, explaining that she has extended her life but in exchange wants the brothers to obtain something known as the Penguin Drum. As Shōma sleeps that night, Kanba ends up kissing Himari as she sleeps.

The Penguin Spirit appears again and tells Shōma and Kamba to board a train and find a girl named Ringo Oginome who may possess the Penguindrum. With Ringo attending a girl's school, Shōma and Kamba send in their penguins to follow her. The brothers end up following her to a lingerie store, where they spot her maneuvering round the outside of a building in order to take a photo of something, which she shows to the brother's teacher, Mr. Tabuki. She then follows him home and hides out under his house, where the brothers conclude that she is a stalker, and pulls out a diary which allegedly fortells destined events.

As Ringo contemplates making a special curry for Tabuki, Shōma and Kamba, having been reminded about Himari's fate if they don't cooperate, break into Ringo's house to search for the Penguindrum. After just managing to hide from her when she comes back home to prepare her curry, the brothers follow Ringo to Tabuki's house, where she finds another woman living with him. Ringo swaps the curry she was preparing with her own and runs off, where she runs into Himari who invites her to her house, much to the brothers' surprise. After eating together, Ringo returns home and confirms the events laid out in her diary.

As Ringo gets help from Himari preparing lunch for a 'date' with Tabuki at the park, Kamba tasks Shōma to go with her and look through her bag for her diary, believing it may be related to the Penguindrum. When they reach the park, they encounter a runaway skunk who runs off after squirting Ringo in stink. After changing clothes, Ringo and Shōma meet up with Tabuki, who brings along the woman Ringo saw him with, an actress named Yuri Tokikago. Come lunchtime, Ringo's prepared lunch gets eaten by crows, leaving her in the shadow of Yuri's picnic. In order to fulfil a 'first kiss' note in her diary, Ringo plans to jump into the pond and have Tabuki save her. However, the skunk appears again and knocks her in, forcing Shōma to save her instead and use CPR on her, though Ringo believes Tabuki to be the one who did it. Later that evening, a girl that Kamba met up with earlier is pushed down the escalator of a train station, completing the description in Ringo's diary.

Asami Kuho, the girl who was pushed down the escalator, is questioned by a woman named Natsume, who then fires a red ball at her head, causing her to remember nothing about the incident. Kamba hears that his uncle is planning to sell their house and goes to see a strange person to obtain the money to buy it. Meanwhile, Shōma and Himari find Ringo, who had a bad day, and invite her back to their place. There, Shōma asks Ringo about her diary, which she claims is a Diary of Fate which describes her future. Both she and Shōma are summoned by the Penguin Hat, who orders Shōma to retrieve the diary, but Ringo breaks free and throws away the hat which lands on a truck, causing Himari to fall dead again. As Kamba goes to retrieve the hat, he recalls what happened 9 years ago when he went with his father into a typhoon when Himari was ill with a fever. After a tough struggle, Kamba manages to retrieve the hat and return it to Himari.

Kamba goes to see Asami only to find she has forgotten all about him. He tries getting some information out of two other girls, but they are attacked by Natsume's red balls, causing them to lose their memories. Meanwhile, Shoma reluctantly follows Ringo's orders in order to let her have her diary, having him move a lot of stuff over to Tabuki's apartment, but she ends up coming down with a fever. It is revealed that Ringo's late sister, Momoka, used to date Tabuki and Ringo felt that, in order to be accepted by her family, she would have to become Momoka by fulfilling what was written in her diary. Despite her fever, Ringo sneaks off to under Tabuki's house to spend her 'first night' with him, whilst Natsume is shown to have her own penguin, discussing 'Project M' with someone.

Tabuki invites Ringo to see one of Yuri's plays with him, followed by dinner, where Yuri invites her and Shoma to a party. At the party, Yuri announces that she and Tabuki are engaged, but Ringo remains determined, dragging Shoma to her school to help her make a love potion, which is foiled by Shoma's penguin. Determined to complete her mission, Ringo sneaks into Tabuki's room one night with the intention of getting impregnated by him.

Ringo attempts to make a move on Tabuki only to find he is not there as his roof had a leak. The next day, Tabuki tells Shoma that he is moving in with Yuri, which does not settle well with Ringo, who later discovers her father consorting with another family. Undeterred, she visits Tabuki's new apartment whilst Yuri is on tour, using a special cake to drug Tabuki whilst she pretends to be Yuri in order to have sex with him. However, Shoma manages to regain his consciousness and delay Ringo enough before Yuri returns home. As Shoma tries to stop Ringo from taking things any further, her diary is knocked off and is later ripped in half by a passing motorcyclist when she tries to retrieve it. As the shock leads Ringo to walk into the path of an oncoming car, Shoma manages to push her out of the way but is hit himself.

Himari has a strange dream in which she meets a man named Sanetoshi, who shows her various books containing her memories. In elementary school, she formed a group called Triple-H with her best friends, Hibari and Hikari. When she argued with her mother over buying a wrong ribbon, her mother became scarred from a falling mirror, she and her friends tried to kill a koi for its blood to heal her mother. She left elementary school due to her illness whilst her friends went on to become idols. Upon waking up after vaguely recollecting a soulmate she once had, Himari hears from Ringo about Shoma's accident.

Shoma wakes up in hospital with supposedly minor injuries, keeping quiet about what happened between him and Ringo from his siblings. As Ringo explains what happened to her diary to him, Natsume disguises herself as a nurse and kidnaps Shoma, demanding the other half of the diary from Kanba and Ringo in exchange for his life. Deciding the to leave the diary half with Ringo, Kanba goes to find Shoma, whilst Ringo is told to surrender the diary herself. After luring Kanba into a trap and kissing him, Natsume escapes but is confronted by another person in a penguin hat, whom Natsume identifies as Mario.

Kanba goes to confront Natsume about the memory-erasing balls and why she desires the diary, unaware that she only has half of it. Meanwhile, Ringo is still determined to complete Project M with or without the diary, making a love potion to give to Tabuki, which actually seems to work. As Ringo decides she can't go through with it, Yuri helps her come to the realisation that she is actually in love with Shoma. As Penguin Himari brings Shoma and Ringo into a Survival Strategy meeting, Himari tells them about the day 16 years ago when Momoka died and she was born, which turns out to be the day that Shoma and Kanba were also born.

Kanba goes home and is called by Sanetoshi claiming that Himari will die once more. Shoma goes into detail of what happened 16 years ago, as Shoma and Kanba's parents were high ranking members of a terrorist organization and were responsible for a terrible incident which killed Momoka. Himari still possessed loses her strength and tells Shoma and Ringo to acquire the Penguin Drum and to stop "them". Himari is then transferred to an hospital and Kanba is called by Shoma. He arrives to find Himari lying dead and tries to once more make a pact to give her his life, but it does not work as she says she must leave to "The Destination of Fate". Just as Himari seems to be truly dead, Sanetoshi appears.