From The TimeSplitters Wiki

TS Series

The games in the series all play a part in the TimeSplitters history. What TS really is about is a war between the Splitter and the Human race. The main character, Sergeant Cortez goes with his sidekick Corporal Hart to retrive the Time Crystals stolen by the Splitters. The crystals hold the power to journey through time, and the Splitters were using it to change the history of the Human race to their interest.