Darin is a mysterious young mouse who has strange markings on his forehead, and also exhibits unusual powers. The boy appears to be mute, and he seems to have a fluctuating grasp of the things said around him, often fairly lucid but at times quite oblivious or blatantly naive. His behavior around people and places he feels are safe is calm, unthreatening and curious, but he far from simply docile, and will respond to threats by running or defending himself with full alertness. Where he came from, who he is and any goals he has - if he even knows himself - he is unable to communicate, leaving him both powerful yet vulnerable, dependent on others to show him trust he cannot give thanks for.
Story Summary (Spoilers Beyond This Point)
During a show by the band HamstaPowah, Sarai Kamilia, who is supervising the building's sound and camera security, spots a young mouse boy being chased by a rat security guard. She informs the band's manager, and after a little helpful intervention involving a stage light to distract the guard, the boy uses the opportunity to escape, stunning his pursuer with a sudden blast of energy from his hands. He ends up in the rafters, and loses his balance when another employee surprises him, tumbling down towards the stage. Luckily someone is prepared for this, and his fall is moderately cushioned by the hamster J'Aime Beck.
The band gather around the boy, wondering who he is and where he came from, J'Aime and her companions - Samuel Boyd and Hiaté Noumou - watching from the side. Mark Hamsta notices that he has a serial number tattooed behind his shoulder, suggesting he's escaped from a laboratory, but Tracy Starr tells him to stop scaring the poor young kid. Imagining he likely has nowhere else to go, she adopts him, naming him Darin with a little help from Sarai.
Later on, while the band are travelling through a forest on their cat Bobo, they are attacked by a strange robot as big as the cat is. At the prompting of a mysterious figure somewhere else, the robot tries to capture Sarai, but Darin stops it, displaying his unusual power more prominently than before leading to the robot being ordered to capture him as the original target. He damages the robot but is incapacitated, only the timely intervention of Mark - riding Bobo and stabbing the machine's central eye - destroying it. Sarai runs to Darin to make sure he is alright, and Darin mutters 'Naru... run...' to her, his first words said in front of them, and passes out. The band return to their house where Darin rests off the fight. Mark discusses possible links between a part of the robot he brought back, the lab Darin escaped from, and a mouse in the news who had been 'reverted' to how mice used to be.
The next day, the city of Crosond where the concert took place is invaded by smaller but similar robots, commanded by a massive one. Sam, Sarai, her mother Kerice Kamilia, and HamstaPowah's manager Snowbell Crystal, are caught in the attack, Sam the only one with combat experience and lacking a weapon. Thankfully Darin shows up, destroying one of the robots instantly, and when Sarai yells to him he twitches, then yells out her name, telling her to run. The others escape while Darin holds his ground against a large number of robots, destroying some before finally getting overwhelmed, sedated, and captured.
When Darin comes to, he is inside the main robot with the all other captured civilians in a thick glass cell, the rest of them still out cold. Sam and Hiaté have just managed to infiltrate the machine, seeing more captured people being dropped off. Sam wonders who would do such a thing, and Darin points a monitor inside the machine as it activates, a human scientist remotely addressing them. The scientist, Doctor Lentle, tauntingly greets Darin using his tattooed designation, noting the sedatives failed to work properly on him as usual, Darin showing clear anger while he speaks. Sam demands to know what he wants with Darin, and Lentle reveals his intentions to revert the non-human species to the way they once were, restoring humanity's dominance. As he laughs, Darin escapes the cell with his power, Lentle yelling disbelievingly that it was supposed to be suppressed as Darin utters 'Naru...' and destroys the screen. His power begins to expel violently.
Seeing Darin is going out of control, Hiaté creates a shield to protect them and the unconscious civilians from the expanding destructive force. The power begins to overwhelm her, but Sam steps in and uses his Relay ability to lend his strength to her own, both of them holding out until the growing energy bursts the entire robot into two from inside. Hiaté, Sam and the civilians are protected by the shield as their half crashes to a halt in the city, the destruction of the command robot resulting in the rest of the units shutting down, ending the attack. In the aftermath, Darin's fate is initially unknown, but Mark appears carrying the mouse on Bobo, Darin seemingly back to normal as he comes to.
That night, a strange female mouse standing in a surreal landscape, speaking to someone called 'Zwihei'. She talks about a mark they have that stands for courage, but also rage, and in her opinion, blind stupidity, which she says suits them well. She turns around, and her remarks are revealed to be addressed to Darin, who is kneeling in a bizarre disruption in the landscape. She ponders whether he is not replying of his own will, or if 'that man's concoctions' have rendered him unable to speak.
Meanwhile, Hiaté, Sam, and several others not with the band infiltrate an underground arena in Crosond, looking to recover the Rod of Ulzis, an invaluable artifact stolen from Hiaté. The events end with the entire illegal stadium being destroyed in an immense explosion, the fates of the ones inside unknown, but looking bad. The band, Kerice and Sarai watch news coverage next day, concerned for their friends. They also note Darin is sleeping extensively, and Sarai volunteers to check on him.
Sarai enters Darin's room, finding the mouse missing and the walls and floor covered by crazed, scattered drawings. She looks out the window and shouts for Darin, and he turns out to be up on the roof, hoisting Sarai up to join him with a thump and rough landing. Sarai sees a massive and intricate symbol Darin has drawn on the roof, and as the others arrive in reaction to the thud, Sarai runs after Darin onto it. They vanish in a column of blue light, and Sarai finds herself floating in whiteness.
Sarai drifts through a surreal space, seeing fragmented images and hearing voices - a mouse girl calling herself Naru addressing someone named Zwihei, Darin fighting robots just like the ones who attacked the town, him seemingly unable to control himself. Sarai encounters a strange, weasel-like creature made of light who leads her through further images, identifying them as memories, Darin's, or Zwihei's as is his original name. The creature calls the place they're in 'Liejriel', or a 'dreamscape', saying Zwihei brought Sarai here to show her his past. Sarai notes that she keeps calling him that, so the creature explains that she and her brother bestowed three special marks, one to Darin, who has Zwihei: courage through haste. The marks were designed to be a defense against 'their enemy'.
They reach the heart of the dreamscape, where a powerful memory is kept, and witness Doctor Lentle torturing Darin, asking how he continues resisting efforts to revert him, and burning his older sister, Narunetsume, alive before him to try and break his resolve. The light-weasel says the memory is too painful for Darin, so he makes them leave, and she takes on a human-like form. She identifies herself as the Spirit of Light, and it is revealed that the others on the roof are being shown the same memories they are through the blue column of light, which still emanates from Darin's symbol.
The group puts together what Darin is showing them - that Doctor Lentle is seeking to revert all non-humans species, verified by a memory of his rant inside the robot, which they were not present for. Finally, Darin himself appears before Sarai and asks them for his help, and they are brought back to the roof. Darin passes out once more, exhausted by the ordeal. Everyone turns in, Sarai appearing to have a strange flicker of energy on her body that she hides. Back in the strange landscape with the unnamed mouse, she expresses a detached concern that Zwihei is still silent, admitting that he gives her hope that there are lives worth sparing, including her own. Darin finally responds, the memories around him showing his new friends as he replies a lot of people are making his life worth living.