A delivery man named Taiya Hando suddenly interrupts a wedding at a church, taking the bride, Mira Shifuto, from the church and into his car before driving off with her in the front passenger seat. Before the groom, his father, and his subordinates drive after Taiya in pursuit as it's revealed that Mira was forced into marring the mob boss's son, can completely catch the two, the Great Space Invasion Running Team Hashiriyan appear. Taiya transforms into Bun Red in front of a bewildered Mira to protect her from the foes who've turned Mira's wedding dress, a precious memento from her grandmother, into a monstrous created called a Kurumaju. Ishiro Meita, an informant working for Taiya, later revealed to be Bun Blue, is sent to retrieve Mira while Taiya deals with the threat, however, in a series of odd events, Mira decides to join Taiya as she becomes Bun Pink. After defeating the Kurumaju, it's revealed by Taiya was sent to retrieve her and bring her to the airport to meet up with her actual boyfriend to leave Tokyo. Mira, thinking over Taiya's words to her to "take the wheel", breaks up with her dismayed boyfriend, declaring she's decided to join the Boonboomgers.

Ishiro is still wary about Taiya letting Mira join the Boonboomgers, but the three accept a special mission regarding delivering some special documents to an important politician, which is disturbed by another attack from the Hashiriyans.

Mira found herself a job to deliver a present from her boy to her grandmother. Taiya refuses to help and Mira is forced to make the delivery by bike, when the Hashiriyans strike again.

Mira is arrested by police officer Jou Akuse and brought to Shirabe Saibu, a member of the International Space Agency who wants the Boomboomgers to cooperate with the authorities against the Hashiriyans, which Taiya refuses. Taiya and Shirabe eventually strike a deal, with the ISA giving free rein to the team, in exchange of having Jou act as a liason for them.

Jou is eager to join the other Boonboomgers in battle, but Taiya refuses, implying that he does not have what it takes, but his opinion changes when Jou risks himself to protect the others from danger and he gains the power to transform into Bun Black.

Ishiro is not fully convinced of Jou's character and begins to spy on him, until the Hashiriyans deploy a monster that forces people to show their true feelings and thoughts. Meanwhile, Taiya is attacked by Madrex, which is the leader of the Sanseaters.

Taiya invites his dealer, Genba Bureki to join them in battle as the fifth Boonboomger, but he refuses. However, when Madrex fights the team with another monster, which proves as more than a match to them, Genba decides to reconsider and transforms into Bun Orange in order to turn the tides of the battle.

Taiya reveals his real intentions to form the Boonboomgers, which is to participate in the "Big Bang Grand Prix", the biggest and most important race of the entire universe, while fighting the Hashiriyans was not in his original plans. Mira and Jou feel dissatisfied for not being told about it beforehand, but their worries are cut short when Madrex comes with a scheme to restrain the rest of the team while he has his long awaited showdown with Taiya.

Taiya is seriously wounded after his fight with Madrex and Shirabe informs the team that the ISA decided to have the Boonboomgers disbanded and their equipment confiscated. In the occasion, Bondorio reveals how he and Taiya met and the beginning of their partnership. Meanwhile, Madrex returns determined to settle his fight with Taiya once and for all.

Genba has his first solo delivery job for the ISA. Worried about him, Mira decides to tag along, when the Sunseaters attack the duo.