From the in-game instructions:
For the last week, the newspaper headlines have all been about the daring theft of the Leydon Papyrus, an early Egyptian work on chemical processes. Lara now receives an anonymous commission to retrieve a mysterious relic, the Osiris Codex, from a recently uncovered tomb. Surely there is a connection? The stolen manuscript is the earliest record of ancient Egyptian metallurgy, and Osiris (The Egyptian god of resurrection and eternal life) has always been inexorably linked with alchemy...
Run, jump, climb, shoot and think your way through a variety of levels, watching out for the traps, obstacles and enemies that will inevitably hinder your progress as you travel deeper underground in your search of the fabled treasures of Egypt.