"Cardfight!! Vanguard Gaiden if" is a spin-off of the "Cardfight!! Vanguard" franchise, exploring an alternate timeline where the protagonist, Aichi Sendou, never discovers the card game Vanguard. The series diverges from the main storyline, presenting a world where the absence of Vanguard significantly alters the lives of the characters and their relationships.

In this alternate reality, Aichi remains a timid and introverted high school student, struggling with self-confidence and social interactions. Without Vanguard, he lacks the catalyst that originally brought him together with his friends and rivals. The narrative focuses on how the absence of the game reshapes the dynamics between characters, particularly Aichi, Kai Toshiki, and Misaki Tokura. Kai, who in the main series is a skilled and confident Vanguard player, is portrayed as a loner with a more subdued personality, while Misaki, typically a composed and strategic player, is shown navigating her life without the influence of the game.

The plot unfolds as Aichi inadvertently stumbles upon Vanguard cards, sparking a renewed interest in the game. This discovery sets off a chain of events that gradually reintroduces the characters to the world of Vanguard, albeit in a different context. The series explores themes of self-discovery, friendship, and the impact of seemingly small choices on one's life trajectory. As Aichi and his peers begin to engage with Vanguard, they confront personal challenges and rediscover their potential, highlighting how the game serves as a medium for growth and connection.

The alternate timeline allows for a fresh perspective on familiar characters, emphasizing their development outside the framework of the original series. The narrative maintains a balance between character-driven moments and the strategic, competitive elements of the card game, appealing to both fans of the franchise and newcomers. By reimagining the central premise, "Cardfight!! Vanguard Gaiden if" offers a unique exploration of the "what if" scenario, providing insight into the characters' lives and the significance of Vanguard in shaping their identities.