"Gonna be the Twin-Tail!!" is a 12-episode anime series that blends action, comedy, and fantasy elements, centered around the concept of twin-tailed hairstyles as a source of power. The protagonist, Soji Mitsuka, is a high school student with an intense obsession for twin tails, believing them to be the pinnacle of beauty. His life takes an unexpected turn when he encounters a mysterious girl named Tail Red, who is engaged in a battle against alien invaders known as the UPG. These aliens, led by the enigmatic Ainea, seek to harvest Earth's "Attribute Power," which is derived from human obsessions, including twin tails.

Soji is chosen by Twoearle, a small creature from another dimension, to become a hero capable of harnessing the power of twin tails. Using a transformation device called the Tail Gear, Soji transforms into Tail Red, gaining superhuman abilities to fight the UPG. However, his transformation comes with a twist: he must cross-dress as a girl to activate his powers. This leads to humorous and awkward situations, as Soji struggles to balance his secret identity with his everyday life.

Throughout the series, Soji is joined by other characters who also gain the ability to transform into twin-tailed heroes. His childhood friend, Aika Tsube, becomes Tail Blue, while the stoic and intelligent Erina Shindou transforms into Tail Yellow. Together, they form a team to combat the UPG and protect Earth's Attribute Power. The aliens, each representing different obsessions such as glasses or maid outfits, pose unique challenges that require the heroes to adapt and grow stronger.

The narrative explores themes of identity, self-expression, and the power of personal passions, all while maintaining a lighthearted and comedic tone. The battles are often over-the-top, with the heroes using their twin-tailed powers in creative and exaggerated ways. The series also delves into the backstory of the UPG and their motivations, adding depth to the conflict. By the end, Soji and his allies must confront the true nature of the UPG's invasion and the origins of the Tail Gear, leading to a climactic showdown that tests their resolve and unity.