Unlike the Dragon Ball movies, which don't fit in the continuity, the specials fit comfortably into Dragon Ball cannon. This story takes place in the alternative future timeline that "Future Trunks" comes from. This leads to some compelling story aspects. In this timeline there are no dragon balls, thus making all the stakes a lot higher and all the consequences instantly more dire.
The tone of this special is decidedly darker than the series. The special begins with Goku and most of the Z- Fighters dying. The androids then spend most of their time going on a killing rampage, destroying cities, causing havoc and doing whatever they want.
The problem with most of the DBZ movies are their bland villains being pale imitations of their series counterpart. The villains in this one are fun, charismatic and at times scary. It is a simple concept done to a high degree. What if teenagers got a crazy amount of power, and no sense of responsibility? They would go to amusement parks, fly around cities, and kill anyone that got in their way.
The fights in this are more bloody, visceral and brutal than most of Dragon Ball. There is some great character work for Trunks and Gohan. And the special does a good job of giving a sense of fear and hopelessness for our protagonists.
History of Trunks will go down in history as one of Dragon Balls very best.