I have literally just listened to this audiodrama in an extremely hot bath and I was still wracked with frigid goosebumps as this tale unfolded. Admittedly, I am a huge fan of the Horus Heresy and Nathaniel Garro, which is surely true of anyone who has read The Flight of The Einstein (TFOTE), but it certainly is very good writing and reading.
This is the first we have seen of Garro since TFOTE and without giving much away this encounter involves him starting his new mission as eluded to in TFOTE therefore I would say you really need to have read the Horus Heresy books all the way from Horus Rising to TFOTE and Battle For The Abyss (BFTA), as the Ultramarines and Word Bearers are involved and BFTA really introduces their first conflict. I would also recommend listening to The Dark King and The Lightning Tower by Graham McNeil and Dan Abnett and Raven's Flight by Gav Thorpe, as the former is the first audiobook/ drama in the Horus Heresy series and, while being great it is not as good as this, and the later, which is the first with Toby Longfellow orating and is very, very good. Plus both are set relatively before or around the time of this and came first.
The vocal work is very good, as is the music and sound effects. The best I have heard in the series yet.
All in all this is a great audioplay and those who complain about it's length, as always, are comparing it to a novel. It is a great short story and with all the quality packed in and over 70 minutes of playing time, certainly worth the money.