Ep 10. Thin Red Line

Ichika is a boy who accidentally discovers that he has the power to use an incredible weapon called an Infinite Stratos, the only weapon known to be successful in the fight against the aliens who have been trying to conquer Earth for years. After being enrolled in an all-girls school exclusively for those who can use IS, the girls begin to show more interest in Ichika as they begin to develop their own romantic feelings for him. The Thin Red Line is the tenth episode of the first season of Infinite Stratos.

After receiving a letter from his older sister, Chifuyu, Ichika is on edge and does not seem like himself. When it is time for the class to hit the battlefield, Ichika is their commander, but he is barely able to focus. The enemy is a formidable one, but the girls all of whom are battle ready and reliable are counting on Ichika to pull through. As the fighting intensifies and the girls go out one by one, Ichika's resolve falters. When all seems lost, the girls, all of whom are supporting and counting on Ichika to bring them home, rally around him.

After a long and grueling battle, the girls finally emerge as the victorious team. However, they are all highly concerned about Ichika's lack of focus and determination during the battle. The girls soon learn that Ichika was informed that their older sister, Chifuyu, had gone missing on a mission. Ichika can hardly contain his concern for his sister who he looks up to very much. With the girls' help, Ichika resolves to find his sister.

As the episode draws to a close, a mysterious blonde haired woman in a military uniform is seen standing in Chifuyu's office. She is eager to inform Chifuyu of a newly discovered IS that is said to be extremely powerful. Chifuyu and the blonde woman discuss the possibility of Ichika being transferred to another school in order to hone his skills and protect his sister.