While Ultramarines Captain Uriel Ventris and it's companion Pasanius are on a death oath and counted as dead already, orks appear in one of the planets under their command and sargeant Learchus will lead the defense and erradication of the greenskins.
With this simple premise, Defenders of Ultramar makes for a plain read. Not wanting to have any amazing plot, nor interesting twists, it is just a visual narration of space marines versus orks, very iconic, displaying a lot of unit types, and typical quotes from the rulebooks. That said, it is quite nicely drawn and painted, and it manages to plug itself into existing lore (at least the one existing in the books).
Not a bad comic if you like Warhammer 40k universe, I got mine second hand and I don't regret it :)