Full Metal Panic! Sigma summary: Following the Tuatha de Danaan's seajacking incident, things are back to normal at Jindai High School in Tokyo, or as normal as they get with Sousuke guarding Kaname. After finding out who betrayed Mithril and caused the seajacking incident, Kurz and Mao are sent on a top secret mission in order to capture and bring the traitor back. However, Sousuke and Kaname's lives are about to change when they meet Leonard Testarossa, a high ranking member of a mysterious group named Amalgam. After their meeting, Leonard warns her that he would do anything to recruit her even if it means war in the streets of Tokyo.