Prologue starts Tomb Raider: Underworld, showing Croft Manor in the rain, it then explodes as you begin. You then find Lara in her bedroom having to escape her burning home. Your objective is to reach the living room where you find an injured Zip being helped by Winston Smith. Zip then tries to shoot Lara where she cries out. The screen then changes to Lara on her yacht with the title "One Week Earlier".
After the Thailand expedition you return to Croft Manor and eventually play out the prologue again.
Once you have completed the prologue you get a 25G achievement on Xbox 360.