The last panel of Saikano in Volume 1 had Shuji wishing that Chise had met him so they could run away together. The story picks up the next day, with Chise waiting for Shuji to climb the long hill to school. They don't talk about the night before. Shuji does think about it in this volume, figuring that Chise had a 'job' (what they've started referring to the missions she goes on). The plot during these collected seven chapters is Shuji's past more than anything, specifically, his first love. Chise asks Akemi, who has known Shuji for years, about who Shuji might have loved and Akemi says that Chise should talk to him about it. Akemi then goes to scold Shuji and tell him to open up to Chise, as this possible first love is worrying Chise. (Really, Akemi is my hero. She's such a brash girl.)
The problem is, when Shuji decides to talk to Chise about his first love, an older girl he'd known when he was younger, he sees her - Fuyumi - riding by on a bicycle. This surprises him so much he doesn't say anything and later, when he processes all this, he's afraid to tell Chise as it could hurt her - and she's got enough pain in her life. Unfortunately, the vision of Fuyumi is real and she's back in town. She has married a soldier who is away in the war and misses him desperately. We find out through a series of flashbacks that Fuyumi and Shuji had sex at some point and they were both virgins. Fuyumi invites Shuji to her house and he goes, nearly winding up having sex with her again (her instigation) but he starts crying and leaves. He becomes tense around Chise, not wanting to tell her anything about Fuyumi, especially not what happened at Fuyumi's house. The one time he tries, when he and Chise are on a date, they are interrupted and Shuji loses his nerve.
There is a couple of side stories revolving around the shared diary that Chise and Shuji wrote in during the first volume and they are very telling. Chise is with the soldiers she accompanies into battle and is trying to write in her diary while the soldiers are flirting with her. Their commander, a Lieutenant, appears to be Fuyumi's husband (as he mentions a wife he wants to get back to) and looks very much like Roy Mustang from Fullmetal Alchemist. The soldiers realize that their ultimate weapon is just a little girl after all - and whatever she's becoming, it may soon be too much for them to control, as she manages to obliterate a city off the map during a sudden attack on her.
I am still trying to figure out whether this is a shojo or shonen manga. It has too many elements of both. The humor is more raw than I would expect from a shojo but the romance is too pronounced for a shonen. Whatever it is, it's an interesting mix and I'm almost concerned for the conclusion.