"Heart Gear" is a science fiction manga set in a post-apocalyptic world where humanity has nearly vanished, leaving behind advanced robotic beings known as "Gears." These Gears, designed to resemble humans, continue to function in the absence of their creators, maintaining the remnants of civilization. The story centers on Chrome, a young, amnesiac Gear who awakens in a desolate wasteland with no memory of his past. His only clue is a mysterious pendant he carries, which holds significance he cannot yet comprehend.

Chrome encounters Roue, a resourceful and determined human girl who scavenges the ruins of the old world to survive. Despite the rarity of humans, Roue is fiercely independent and initially wary of Chrome. However, the two form an uneasy alliance as they navigate the dangers of their environment. Their journey is driven by Roue's quest to find her missing father and Chrome's desire to uncover his origins. Along the way, they face hostile Gears, rogue machines, and the remnants of a society that once thrived but now lies in decay.

The narrative explores themes of identity, memory, and the relationship between humans and machines. Chrome's gradual discovery of his purpose and the truth behind his existence becomes a central focus, revealing connections to a larger, more complex history involving the fall of humanity and the rise of the Gears. The world-building is intricate, with the manga delving into the mechanics of Gears, their societal structures, and the ethical implications of artificial intelligence.

Key characters include Chrome, whose innocence and curiosity contrast with his latent abilities and mysterious past; Roue, whose resilience and determination highlight the struggles of surviving in a world dominated by machines; and various other Gears and humans who either aid or hinder their journey. The story balances action, mystery, and emotional depth, creating a compelling narrative that examines what it means to exist in a world where the line between human and machine has blurred.