This isn't a perfect story by any means -- it combines some of the flaws of the new series (overly hurried plot) and old (laughable special effects). Still, underneath those surface flaws there's a crackling good story. The Doctor and his companions return to 20th century Britain so that Tegan can visit a relative. There they find that a local bigwig is turning a harmless village festival, a reenactment of an English Civil War battle, into something altogether more sinister. The Doctor and his companions have to figure out why, and put a stop to it, but to do so they must defeat the very people they're trying to save. Plenty of room for drama there.
In many ways this story foreshadows what Doctor Who would become in the 21st century. The short duration of the story, modern setting, and family links would all become hallmarks of the new series. (Admittedly the contemporary settings, while not part of the show's original concept, had been a feature of Doctor Who since the mid-60s).
If the story had just had a bit more room to breathe and grow it could have been a classic. If the 80s production team had recognized the limits of their budget and focused on special effects with a timeless quality (largely by hiding things as much as possible and letting audience imagination do the work instead of relying on computer graphics which would not age gracefully) that would have helped too. As it was, we're still left with a fun hour of fairly straightforward adventure, and the flaws are forgivable. After all, if you can't stand weak special effects, why are you watching Doctor Who in the first place?