The year is 2000, in the closing days of the First Alien War.
The covert multinational taskforce known as the Extraterrestrial Combat Unit - or X-Com, for short - have discovered the location of the facility from which the Alien forces are overseeing the attacks upon Earth.
This facility, codenamed "Cydonia", must be destroyed if X-Com hopes to win the war and end the Alien threat, and so elite forces are dispatched from X-Com facilities around the world to take the fight to Mars.
Despite suffering heavy casualties, the X-Com troops breach the alien base and manage to destroy the alien's main command-and-control computer, an organic system nicknamed the "Mother Brain".
Without the Mother Brain to direct their operations, the Alien forces fall into disarray and flee from Earth's solar system.
But all is not well - the destruction of the Mother Brain triggered a contingency program built into Cydonia's secondary systems, one which sends a narrow-beam tachyon signal to a waiting receiver.
The destination of the signal?
Forty years pass. All is quiet in the intervening time . . . but then the unthinkable happens.
Alien forces secreted on the seabed millions of years ago - forces reactivated by the Cydonia signal - begin to wage a campaign of terror against Mankind. Strange flying submarines armed with hideously powerful weapons begin staging attacks on human vessels and ports, with terrifying efficiency. As a clear pattern of Alien activity is established, the forces of X-Com are reactivated to counter this new threat.
A small team from an X-Com surface base in the Canary Isles is dispatched to the scene of one of the most recent attacks - the sinking of the Euro-Syndicate cruise vessel Hyperion
Only one transmission is made before all contact is lost with the team:
"Commander? I think they're back . . . "