These three audios feature Tom Baker and Louise Jameson as the Fourth Doctor and Leela, following on from Last of the Colophon.. I'm noting them quickly here so as to move on to better stuff - none of them quite hit the mark for me.
Destroy the Infinite, by Nicholas Briggs, is quite important in Big Finish continuity in that it introduces an enemy entity, the Eminence, which had already featured in Sixth Doctor and Eighth Doctor stories, here having nefariously taken over a human space colony by means of the Infinite Warriors (who, per the title, must be destroyed). A stellar cast, who I won't list here, but somehow the whole thing falls rather flat, and writing a month or so after I listened to it I find I cannot remember much about it. You can get it here.
The Abandoned, by Louise Jameson and Nigel Fairs, was the best and most ambitious of these three. It's set entirely in the Tardis, and foregrounds Leela much more than usual (hardly surprising given who wrote it); I very much liked her getting drawn into an ancient mystery concealed in the depths of the Tardis, and some of the soundscape and mindscape concepts that the story contained (including a great guest performance by Stephanie Cole (who co-starred with Louise Jameson in Tenko may years ago). My biggest problem with it is that I was not entirely sure what was going on, even after listening to it twice. but ambition is not a bad thing. You can get it here.
Zygon Hunt, again by Nicholas Briggs, is a bit more memorable than Destroy the Infinite, but again rather pulls its punches; since you know that at least one of the cast must be a Zygon, it's fairly easy to work out who it is, and the plot unwinds without a lot of help from the Doctor or Leela. The plot itself concerns the loathsome hunting practices of the Knight Defenders of Earth, killing big things for the fun of it, and the crucial dynamic is supplied by the commander, played by Michael Maloney (who I must have first encountered in Earthsearch back in the day), and his adjutant, played by Gillian Kearney who I absolutely loved in Sex, Chips & Rock n' Roll. You can get it here.
Next up: some Seventh Doctor stories.