A brother and sister both in their teens watch their Father die. He was a Police Officer.
Aliens have landed on Earth and are killing; destroying both people and buildings. Considering their size, they are doing serious damage.
Thom and Rachael had heard of what their Father called “decommissioned” walkers and had vague memories of seeing them in their early childhood. They go to find them.
They come across a vast junk yard of ex-police vehicles and equipment and row after row of the so-called walkers. High, enormous cargo-container-like lifters. Four wheels, sixty metres high thin, steel legs. They have the effect they can move around any other vehicle.
They see other sizes; smaller at twenty-five metres high and look at each other.