This is the final and concluding volume to the Full Metal Panic! storyline and the saga of Sosuke and Kaname.
As Mithril is about to execute a final last-ditch effort against Amalgam, they find out that Amalgam has also captured a military base on the opposite side of the world: there they have stolen several nuclear warheads and have set them to detonate at the same time as Leonardo's final plan comes into fruition. An already terribly weakened, under-supplied and under-staffed TDD is forced to divide its forces in order to try and combat both threats - both missions essentially become suicide missions.
Here we see the final showdown between Sosuke and Leonardo, and Sosuke having to face the only man that Sosuke ever thought of as a father, in order to decide the fate of Kaname and the entire world - both past, present, and future.