As part of the organization Mithril, Sergeant Souske Sagara, Melissa Mao and Kurz Weber receive a secret mission: guarding Kaname Chidori, a popular student from a Tokyo high school who has hidden military potential.
Kaname Chidori’s one of the most popular girls at her high school – unfortunately, it’s her growing popularity off campus she should be worrying about. Unbeknownst to Kaname, terrorists are plotting her abduction, believing she possesses the rare and coveted abilities of “the Whispered.” That’s where Sousuke Sagara enters the picture. He’s a hotshot soldier from the clandestine counter-terrorist organization known as Mithril – and he’s going undercover at Kaname’s school to try and keep her safe. He may be an ace in the cockpit of an Arm Slave mech, but there’s no training in the world that could prepare him for the warzone of high school.
As part of the organization Mithril, Sergeant Souske Sagara, Melissa Mao and Kurz Weber receive a secret mission: guarding Kaname Chidori, a popular student from a Tokyo high school who has hidden military potential.
Sousuke is set on his mission to protect Kaname, and attacks anything vaguely resembling a threat to Kaname. This leads to funny situations and Sousuke has to explain Kaname why he is following her, blaming coincidence.
Sousuke gets oriented in his Garbage Duty and Kurtz gets to know the targets a little better. Meanwhile Tessa takes care of a research center in Russia. Sousuke's team encounter a mysterious man climbing to Kaname's balcony.
The airplane with Sousuke, Kaname and 400 of their classmates is hijacked and brought to a secluded location. Kaname is taken away from the plane, but what will Sousuke do to rescue her?
Sousuke comes to Kaname's rescue, and the rest of the rescue team arrives on the scene as well, but can they hold off the army and save the students? Can Sousuke get Kaname to safety?
As the rescue team has left, Sousuke, Kaname and Kurz try to figure out a way to get back to safety.
Sagara and Kaname barely escapes from the enemy. Now on the run they try to find a way out. Deep in the forest Sagara isn't able to move much more with his injuries, so Kaname decides to stop and rest for a bit. After Sagara was able to stop the bleeding, Kaname realizes how dangerous and mysterious Sagara really is. To make matters worse they find an unexpected person along the way, it's Kurtz, who looks badly beaten by Gaul. As the 3 try to make to the shoreline they find themselves in a tight pinch; the forest ended and all there is now is plain fields. Not Knowing what to do Sagara had only one idea left - to have himself and Kurtz distract the enemy force while Kaname makes a run to safety. But Kaname refuses to leave.
Sousuke suddenly decides to become Mizuki’s boyfriend! The cold, watchful eyes of Mizuki's friends settle firmly on Sagara - but it’s Kaname’s reaction he should be worrying about.
When Tessa rushes to an SDF base after hearing that an A21 member has been captured, she’s shocked to find Takuma Kugayama. Seina, at the controls of an RK-92, has an even bigger surprise waiting.
Kaname is in a bad mood after seeing Tessa emerge from the bath. Unfortunately, even as Sousuke Sagara tries to explain that things aren’t what they seem, Tessa finds a way to make the situation worse.
Kaname and Tessa are taken away! The gang follows the signal transmitted by Kaname’s bracelet to find them, but upon reaching their destination, they’re surrounded by the authorities.
The giant AS - Behemoth - is on the move, crushing Mao's M9 in only seconds! When Kurz attempts to engage the monstrosity, the situation looks bad - until a crusader appears in the sky!
Tessa and Mao decide the best way to settle a big argument is with an AS battle! While Mao is calm and composed, Tessa desperately seeks training from Sousuke.
Kaname, Sousuke, and the gang use summer break as an opportunity to visit the Narushino military festival, and during the annual AS competition, Sousuke helps give the underdog an advantage.
After learning that Gauron is still alive, Sousuke is ordered to join the Gauron Assassination Team. As he prepares for the mission, he has no idea the effect his actions will have on Tessa and Kaname.
The Mithril assault team finally begins to take Sousuke's advice into account, but it may be too late: Gauron has one of Sousuke's old comrades predicting what Sousuke will do.
Zaied outwits Sousuke's team and interdicts their advance! Zaied's team surrounds Sousuke's team who is carrying a nuclear warhead. Zaied moves first. While Sousuke is fighting against Zaied, Gray and the team are aghast at the power of Gauron.
Sousuke spends most of the budget for the school festival. Kaname gets so disappointed. It might have been his apology? He invites her to "go to a southern island", but the destination is not what she is thinking...
Flashbacks of Kurz and Sousuke's time as trainees.
The De Danaan is heading toward Melida Island when it receives an emergency contact. Gauron's troops have appeared at Beridaobu Island! This island is the US military's chemical weapons base. They have Gauron's new AS, Venom, rampaging around and also the latest mass production model, known as Shadow. Things start to get tense for Sousuke and the others.
Gauron manages to escape thanks to the betrayal of two soldiers. Gauron takes Kaname hostage and takes over Dana, the AI of the De Danaan.
Trouble brews on the Tuatha de Danaan. Gauron has taken control of Dana and isolated the crew into the back half of the ship. Tessa and Kaname are held prisoner on the bridge, helpless to stop Gauron.
Kurz and Sousuke, free to roam the mostly empty ship, fight to free the contained crew. Meanwhile, Gauron is getting desperate.
Kaname holds the ship together while Gauron and Sousuke face off. The problem is that Gauron knows he is beaten one way or the other. He plans to take everyone down with him.
