From the publisher's website:
Time travelling, adrenaline pumping, sweaty group action. Discordia Incorporated accepts full responsibility for any delirious fun experienced whilst playing Shifter.
From the game's introduction:
Shifter is a role playing game based around a society in the far future so improbable that they have to send interdimensional demolition experts back in time to ensure that it continues to exist. Travelling between different versions of reality, these Shifters stabilise and destabilize different timelines - known as variants - by tampering with history. Along the way, they come into conflict with Rogue Shifters, and Shifters sent from different timelines, bent on eliminating the players' version of history entirely.