Hard sci-fi writer (and scientist) Reynolds takes a turn at the Doctor during the era when the polarity of science was at it's most reversible and does a damn fine job of it. He's obviously having a frankly ridiculous amount of fun playing with the third Doctor, Jo, the Brigadier and the Master.
As always during this time the Earth is being invaded by a vicious alien race, The Sild, who are appearing in force from the future. Alongside this the various members of UNIT seem to be developing a problem with remembering the existence of the Master.
In their efforts to stop the invasion the Doctor and the Master head off into the far future only to discover that they themselves may actually be somewhat to blame for the situation.
It's all excellent fun. It's a big budget version of a no budget TV show and is a blast all the way to the final page. I love the way he's kept to the spirit of the age but has widescreened the whole thing. The vibe is right, the characters are right and the whole thing absolutely romps along.