Although the Great Intelligence plays an important role in this story it's not really the enemy as such. Without wanting to give too much away this sees the Brigadier and the Fifth go up against the mysterious Vault. We finally find out who the General is and what he's up to. Simon's real identity and aims are also revealed and there's significant events for the Travers family too.
I have to admire the way the author (and man in charge of the series) really takes control of things here. There's a lot of characters to manage now but I felt all were well dealt with. I also liked the way minor events in previous books all come together here to give that series finale feel. The author also does a good job at occasionally filling in gaps- there's a hint at why Anne Travers never appeared in 70s Who despite playing a large part in these books and an explanation about what HAVOC actually means in this series (if it appeared before I missed it completely).
If I had to criticize anything it's that this requires a lot of background knowledge. If you don't know the Doctor Who stories 'The Abominable Snowmen' and 'The Web of Fear' or haven't read all previous ten books in the series there will be areas here where you are missing something. At the very least The Forgotten Son, The Schizoid Earth and Times Squared are all essential reading before this. It wasn't a problem for me but this wouldn't be a good jumping on point for new readers of the series.
I seem to be saying this after every book in this series but it continues to get better and better.