Muv-Luv is a collection of the first two entries in a trilogy of visual novels. The first part, Muv-Luv Extra, is basically a slice of life harem anime on the surface. The sequel, Muv-Luv Unlimited, is an epic, mech-filled sci-fi story. Make no mistake, while these two narratives are seemingly-unrelated, they’re very much two sides of the exact same coin.
Source: Destructoid Review: Muv-Luv