If I were to summarize the story, it would boil down to "don't piss off Ultramarines." There are plenty of badass moments for the Ultramarines, including Guilliman hanging out in space without a helmet for a while.
In addition to the Ultramarine heroics and epicly described visuals (like a starship crashing into a city from orbit), there are a few glimpses at Oll Persson, a perpetual and friend of John Grammaticus from Legion. As described in the Lexicanum article, Oll Persson has a role to play in the Horus Heresy, including saving the Emperor's life.
To me, something I really liked about the book was that the Ultramarine characters are characters and not flat, boring, exemplars of 'proper' action as they are so often considered. Captain Remus Ventanus, who appeared in Age of Darkness serves as the lead Ultramarine on the ground. He cobbles together a fearsome resistance, even though he begins the attack on Calth away from his Company. Along the way he picks up a company standard, which he impressively uses as a weapon several times and carries with him throughout the battle.
Another interesting character is Sergeant Aeonid Thiel, who at the time of the attack, was awaiting possible censure and punishment from Guilliman himself for daring to offer theoreticals on fighting fellow Astartes. As the battle breaks out, he finds himself in Guilliman's trophy room, commandeers some relic blades and proceeds to lead the purge of the Ultramarine flagship. In the end, Thiels heroism leads to the origin of red markings on helmets to signify squad leaders, turning a mark of censure into a mark of honor.
The book also gives a few glimpses to Tylos Rubio, who during the fight (though not shown in this novel) is approached by Garro and recruited into the Sigilite's personal warriors, as depicted in the Garro Audio series.
Again, like it my last review, it has been quite a while since I read the book. However, like many of Abnett's books in the series, Know No Fear combines some awesome heroics with cool tie ins and backstory to the Heresy. As such, I would definitely recommend this one as a good read.