On an alien planet in galactic sector 973 lives a race which wants to bring peace to the galaxy by establishing a united galactic empire - of which they are the rulers. In order to accomplish this, of course, they needed the most disciplined and courageous fighters. And what better way to weed out these skilled individuals than by having them participate in war games?
With this in mind, the aliens - led by a War Lord - set up eleven triangular war zones over part of their planet, with their base in the very center. These zones were separated from one another by a mist-like barrier, through which the combatants are unable to cross - some kind of psychological imperative keeps them from continuing further through it.
Into these zones, the aliens chose to place human troops. They removed human specimens from their own time and place and put them in a practically-identical context within the war zones, where they continued to believe that they were living their own lives and fighting the same war. The process of being brought to the aliens' planet seems to them like suddenly being surrounded by mist; when it disappears, they are in their war zone.
The humans are then placed in mental processing machinery which erases their memories of how they arrived on the aliens' planet. It also causes them to be unable to see objects that are beyond their comprehension; everything else, they rationalize. The sterile rooms of the aliens' base is perceived as whichever war zone the human has come from. In the zones themselves, the humans are kept subjugated by special glass used in spectacles or monocles, which seems to have a hypnotic effect when the wearer speaks.
The war zones assembled by the aliens seem to be the World War I zone, the Roman zone, the American Civil War zone, the English Civil War zone, the Mexican Civil War zone, the Crimean War zone, the Russo-Japanese War zone, the Boer War zone, the Peninsular War zone, the Thirty Years War zone, and the Greek War zone.
While the wars rage on, the aliens sit in their War Room, outfitted with maps for each zone, and plot what strategies they will use against one another. Then, they return to their assumed roles as leaders of their human troops to execute their manoeuvres. While incognito, they can communicate with the base through the use of video terminals which are scattered throughout the war zones.
If the aliens need to return to their base, they make use of large green space-time capsules known as SIDRATs. Similar in design to TARDISes, and also indestructible, they are also equipped with dimensional flexibility and remote control - although these additional features have resulted in a shorter lifespan for the vehicles. The SIDRATs they possess are a gift from their War Chief, a Time Lord renegade and traitor who took several machines with him when he joined the aliens' cause. He plans to take over the galactic empire once it is formed - but because his SIDRATs are running out of energy, he hopes to use the Doctor's TARDIS to secure his victory.
The aliens' base is set up as a training school, to demonstrate how their techniques are used. Lectures are held in some of the rooms; one is used for security; and another is the aforementioned War Room. Among their technology is a helmet with a long eyepiece which can be used to make people tell the truth, display photographs, revive a stunned human, or destroy their mind. They arm themselves with long-barrelled guns that can either stun or kill. They themselves seem to be sensitive to bright light.
Unfortunately for the aliens, their mental control can be overcome. Also, a five-percent failure rate in their mental processing causes some humans to shake off their conditioning and form pockets of resistance in the various war zones. When the resistance cells are organized, they are able to destroy the aliens' videophones and then cause a great deal of havoc in their central base. The Doctor calls the Time Lords in to help him return the captured humans to their own times, and when the War Lord is put on trial, he is sentenced to dematerialization - it is as though he never existed at all. His home planet is surrounded by forcefields, forever isolating it.