From MangaHelpers:

This is another story in the Full Metal Panic universe, set some years after the events with Sousuke and Kaname. Ichinose Tatsuya is a pretty normal high school boy, who lives with his father and little sister Yukari. He has a tendency to fall in love with foreign women, which has never turned out well for him. One day, while up in the mountains driving a Power Slave to help out with his father's company, he runs into yet another beautiful foreign woman, called Adelina. He lends her a hand and is surprised to see that she has a military Arm Slave, called "Shadow," with her. What is this mysterious and highly armed woman doing up in the mountains? Tatsuya's life is about to get a lot more exciting... [tethysdust]