Yu and the the squad make their way to Shinjuku in time for the city's defense against vampires.
Meanwhile, Guren identifies the invasion force's leader - Ferid Bathory - and proceeds to eliminate him with his forces. He fails to eliminate Ferid, and is instead brought to a brawl with Mikaela Hyakuya.
As Yu and the squad make their way to assist Guren, Shinoa reminds them of using a drug that could help them increase their strength to fight vampires, but warns that an overdose could kill them when they use the drug.
Amidst the ensuing struggle, Guren is overpowered by Mikaela and Ferid (who interfered and struck Guren), and as he was about to take a second, possibly lethal, dose, Mika thrusts his sword at Guren. Immediately afterwards, Yu sees the situation and charges at Mika with his sword, but at that moment both of them recognize one another.
List of characters in order of appearance:
- Mikaela Hyakuya
- Yūichirō Hyakuya
- Mitsuba Sangū
- Shihō Kimizuki
- Yoichi Saotome
- Shinoa Hīragi
- Crowley Eusford
- Guren Ichinose
- Ferid Bathory
- Shigure Yukimi
- Norito Goshi
- Sayuri Hanayori
- Mito Jūjō
Four years ago, Mika and Yu are hanging out with the other children, and Mika asks Yu about escaping the vampire city. Yu tells Mika they would never get out, but Yu admits it would be cool if they and their family escaped. Much to Yu's confusion, Mika says that he will try harder after all.
In the present, Shinoa Squad is on the battlefield in a heavily damaged area with numerous corpses of both human and vampire soldiers scattered left and right. The squad slaughters vampires and heads off to meet with Guren at Shinjuku's 5th Street intersection.
Shinoa surprises the squad newbies with a "very first training session." She explains the concept of drugging up to drastically raise one's synchronization with a demon for 15 minutes. One tablet raises power to 150% while two tablets raise it to 180%, but two may kill the user by sending him/her into shock. Three tablets will rupture all of the user's internal organs. After the 15 minutes are up, the synchronization drops to zero, and the user becomes utterly defenseless. In order to avoid this drawback, the soldiers carry a timer. This timer goes off after 13 minutes, signaling for the soldier to flee to safety before becoming extremely weak. The supplement requires 10 seconds to become active, which is why they did not have time to use them when they encountered Thirteenth Progenitor Crowley Eusford in chapter 10.
The side effects are extreme, leading Shinoa to joke, "The Demon Army itself one hundred percent guarantees that these supplements are positively riddled with harmful side effects." This unnerves the boys, but they each take a packet of the pills anyway.
Mitsuba and Shinoa explain that the ranking officer at the battlefield gives the orders to take these drugs, but most prefer to avoid them unless necessary.
At the 5th Street intersection, Guren and Mika clash. Ferid warns Mika about his overwhelming arrogance, but Guren and his squad have already managed to lure Mika into a trap. Guren nearly manages to stab Mika from behind when Ferid interferes, backhanding Guren and sending him flying while claiming that Mika owes him one. Mika rejects Ferid's request to fight together and he orders his sword to drink more of his blood.
Guren contemplates taking a second tablet. Mika asks if he is done talking, and Guren asks if he can have another 20 seconds to take his medicine. Mika answers by attacking with greater strength and speed than before. As he prepares to make the finishing blow, Shinoa's squad appears and takes their drugs.
Mika stabs Guren through the chest. Yu arrives to protect Guren and promptly stabs Mika through the chest. Yu curses Mika, saying, "Die, Bloodsucker... Huh?" when their eyes meet.
Manga panels provide a visual overview of the events of chapter 12 with depictions of the characters, Seraph of the End world and scenarios.