The Doctor, Jamie and Zoe find themselves in a WWI style battlefield. Bombs going on all over the place, trench warfare, etc. They're brought into a British trench on the front lines and of course thought to be spies, especially as they couldn't explain how they (civilians) got there. They were handed up to the local General who conducted a kangaroo court to convict them as spies. It eventually developed that the General was not quite what he seemed to be, but instead was an Alien whose intent was not understood until the end of the story.
After some go arounds they escape from the British and start exploring the area around them. One thing they discover was a band of mist, and driving through the mist brought them to an area inhabited by soldiers of Ancient Rome. Hmm how curious, going from WWI to Ancient Rome. They also later found an area resembling the American Civil War.
It eventually came clear that these Aliens had collected warriors from Earth's history, put warriors from each period into their own region, and were conducting a cruel sort of experimentation. They had an oddball goal of developing an army capable of conquering the Universe, using an army of mind-controlled soldiers collected from human history.
It eventually developed that among the aliens was another Time Lord. This Time Lord was one of the early few seen, other than Susan (the First Doctors Granddaughter) and the Wandering Monk in the Time Meddler. Like the Doctor this Time Lord (known only as The War Chief) was a renegade having escaped from their home planet. Unlike the Doctor this one was pretty darned Evil. It's tempting to think this guy was also The Master, but there are several behavior differences that make it clear this renegade Time Lord was not the Master. Besides the War Chief died towards the end and seemingly not regenerated by the Time Lords.
This story includes the first view of Gallifrey. Towards the end of The War Games the trouble was so large scale he couldn't handle it himself, and had to turn to the Time Lords for help. But that came with a cost, as we learn he had escaped from Gallifrey under unfriendly circumstances. These Time Lords are very unlike all following implementations of the Time Lords.. e.g. none of the fancy headgear. In any case they put him through a trial, decide to exile him to Earth to work with U.N.I.T., but in a new body. This was the Doctor's second on-screen regeneration and unlike most of the others this one was not due to death, but due to the Time Lords forcing him into regeneration.
The video on the DVD is excellent all the way through. The extra's on the third disk are a great addition giving some interesting background material.
For example that the story was created under a huge time pressure. They had had two (or three) other stories lined up for the final 10 episodes but those stories failed. That forced them to create a single story over 10 episodes, and that they were still writing the story as the final episodes were being filmed. One would think a rushed story would come out badly, but in this case it worked excellently.