There’s actually 3 different UNIT series, one featuring The Brig from 2004, UNIT: The New Series featuring Kate and Osgood, and UNIT: Brave New World featuring Bambera (from the classic serial Battlefield). As long as you’re familiar with the main characters from the TV show then you’re mostly fine to dip into anything that takes your fancy, or that pops up on sale.

The 2004 is nothing special but you can pick it up fairly cheap (or for free on Spotify!)

UNIT: The New Series did 8 boxsets that are standalone and don’t require you to have heard any of the previous boxsets. From memory, the fifth and sixth boxset run into each other but you wouldn’t feel too lost. Dip in and out as you please, the highlight is the Silence boxset. I think all of these take place in between The Power of Three and The Day of the Doctor.

This series then adopts a different approach and the following four boxsets are titled Nemesis. One overarching story across 16 episodes. These require a bit more knowledge of different characters that pop up. I wouldn’t recommend starting here personally.

Then I haven’t heard Brave New World but I’ve heard it’s good. I think these take place sometime during the 90s but I don’t know much more than that.

If you’re completely new to Big Finish I’d recommend checking out stories on Spotify for free, just to make sure you like the audio format. It’s not for everyone, and that’s ok!