Captain Nathanial Garro is a loyal follower of the Emperor and leader of the 7th Company of the Death Guard Space Marines. When Warmaster Horus' treachery is revealed on Istvaan III, Garro takes control of a ship called the Eisenstein and sets of to warn the Emperor about what he has witnessed.
James Swallow tells this story with gusto in The Flight of the Eisenstein, which introduces us to Garro and shows us aspects of his character that lead him to wanting to warn the Emperor about the treachery of Horus, and the threat that Horus is to the Imperium. While the Eisenstein is fleeing from orbit around Istvaan III it is fired on and sustains dangerous amounts of damage, despite this it jumps into the warp where the Chaos Gods begin to play with the ship. Garro and his men must survive if they are going to warn the Emperor, but the longer they stay in the warp the less likely it is that they will be able to.
Swallow does a good job of raising the tension and putting Garro through the wringer. If Garro succeeds maybe the Imperium can defeat Horus, but if the Eisenstein is lost in the warp Horus will most likely win. A good entry in the Horus Heresy series that focuses on the Space Marines themselves rather than the Primarchs for a change.