While the war rages on across the galaxy, for many on the Throneworld of Terra it exists only in rumour and hearsay. But in the course of her daily duties, humble adept Katanoh Tallery discovers evidence of a conspiracy within the monolithic Administratum - surely, this can only be the precursor to a traitor attack on the Solar System itself?

Fleeing into the shadows, she soon finds herself at the mercy of the mysterious Knight Errant Nathaniel Garro. Will this stalwart defender of mankind be her saviour, or her executioner? It's the start of a brand-new chapter for Garro. He's gathered Malcador's Knights Errant, and now he embarks on a personal quest to discover his role in the wider Horus Heresy - and in doing so he discovers a secret that could change the course of the war.