East and West battle with magic during the siege of Vienna in 1529. Brian Duffy is a sword for hire living in Venice. After a brawl with some locals he runs into a man named Aurelianus, who offers him a job in Vienna, a city which Duffy departed three years earlier.
The job is as a bouncer at the Zimmerman Inn, home of the world famous Herzwesten beers. Duffy agrees, partly for the money and to avoid the locals he was fighting.
As he travels to Vienna he runs into odd people and is attacked by bandits, narrowly escaping. He has weird dreams and hallucinations, and wonders if they are due to his years of drinking. But he's convinced of their reality after a host fantastical beasts escorts him safely across the Alps. He begins to wonder why a Viennesse publican would hire a bouncer in Venice.
He finally arrives in Vienna along with a fellow traveler named Bluto, who has also been hired by Aurelianus as well, in his case to organize the city artillery. He gets to the Inn and presents his letter of hire to the reluctant manager.
Meanwhile the Turkish Sultan is conquering Europe and has his sights on Vienna. Soldiers and mercenaries are pouring into the city, along with a number of odd individuals that Duffy can't help but notice gravitate to the Inn.
A month passes during which Duffy keeps the peace at the Inn. One night he spots an explosive left at the brewery door. He throws it outside and in the ensuing commotion he flies into a strange rage and speaks like a different person altogether.
The next day he visits Bluto to find out if the gunpowder in the bomb might have been stolen from his artillery. The two discuss who would want to destroy the brewery in the first place, since it has no rivals.
As Easter, and the end of his contract, approaches Duffy considers leaving the city, but then Aurelianus arrives back at the Inn. Duffy tells him about all the strange sights on his travels and demands to know what is really going on. Aurelianus demurs but tells him that he is here to help fight the enemies of the West.
Duffy assumes he means King Suleiman and the Ottoman army but Aurelius clarifies that he means Ibrahim, the Grand Vizier, who is actually half-demon and half-human and a great magician in service of the mystical Eastern King - the opposing force to the King of the West who rules above the human realm.
A viking longship sails up the Danube into the city and crowds gather to gawp, Duffy joins them and is mistaken for the legendary hero Sigmund. Aureliuanus is not surprised by this, and invites the Vikings to join in the city defenses.
Later Duffy is attacked by two flying demons, and feels possessed during the fight, as though someone else is controlling his body. Aurelianus reveals that Duffy was experiencing a previous incarnation which took over because it recognized the beasts. Duffy asks him who he used to be but Aurelianus won't tell him yet.
Duffy becomes increasingly frustrated with Aurelianus, who is clearly trying to manipulate him for his own motives while keeping secrets. The wizard offers him a promotion, to bodyguard of the King of the West ,who he now refers to as the Fisher King, causing a weird feeling of deja vu in Duffy who nevertheless accepts.
One night he spots a vat of beer he hadn't noticed before, a Dark brew to go along with the two others normally served at the Inn. Aurelianus tells him it is the original beer brewed these for three and a half millennia and it is only drawn every seven centuries. The beer has potent magics and will heal the Fisher King and restore his land to health at the same time. But it can't be drawn until All Hallows Eve, six months away, and that is why Ibrahim needs to attack the city, to destroy it.
Finally Aureliuanus reveals the full truth; sensing an approaching crisis the Fisher King asked to gods to send a hero, the reincarnation of King Arthur himself. Aurelianus spent twenty years following the portents before finding him in Venice. He gives him Excalibur as proof, and Duffy remembers and believes.
Duffy experiences both sets of memories, and unwilling to let his past life take over his new one he panics and tries to escape the city but finds himself unable to will himself to go. Instead he goes with Aureliuanus and an escort of Vikings to get the Fisher King from the woods outside the city. They transport him back to the city in a wagon, and survive an attack from Eastern supernatural forces along the way, in part thanks to Arthur taking possession of Duffy again.
By September King Charles sends 85,000 soldiers to defend Vienna, and they arrive just three days before the Sultan's army. The city is now under siege and the bombardment is constant. Duffy, back in control of himself and his mind except for the occasional dream, helps fight on the walls against the Turks.
As the danger grows, more of the mystical types who have been drawn to the city for the drawing of the dark become impatient and demand that it be tapped before it is too late and Ibrahim gets his hands on it. Aurelianus refuses.
The wall is briefly breached by the Turks and Duffy joins the defenses, beating them back with massive casualties to both sides. Some of the magical types attempt to rush the Inn, convinced the city will fall soon, to get to the beer but Duffy as Arthur fights them off.
The two sides are at an impasse, the conventional armies both exhausted and the two magicians evenly matched, with equal counter-spells that prevent any big magics from being used, Then Aurelianus finds someone has stolen a small book of simple hedge witch spells, including one that can spoil beer. It is weak and can only be used at noon or midnight, but can fly under the radar of his defenses.
Aurelianus knows that Ibrahim will have to come close to the city personally to use the spell, so he decides on a dangerous gambit; bringing the Fisher King into battle with them, which will shift the balance of power but risk everything by putting the King in physical peril.
Duffy and the Vikings take the Fisher King over the wall and sail to where Ibrahim is preparing the spell. They carry the King to the riverbank and as soon as he is officially on the field of battle they feel Ibrahim's magic subside.
With the countermeasures gone, the landscape seems to shift and new ships pull up; Roman, Greek, Phoenician and others, occupying more space than should be possible. On the Eastern side similar forces gather. They then witness a battle between the magical forces. With the Fisher King in play the Western side prevails, although Ibrahim escapes.
Duffy is befuddled from the sorcerous battle, during which the part of him that was Arthur "died", having completed its duty. He falls into the river, injured, and Aurelianus rescues him.
With Ibrahim routed Aurelianus' full powers are restored, and though the Turkish army is still outside, he knows the beer is safe, and thus so is the Fisher King and by extension the West.
With his blessing he lets Duffy and the Vikings depart the city and they sail down the Danube in the longship.
Best part of story, including ending:
It's a fun look at historical events through a mythological lens, the idea that a real clash of magic armies was actually just the surface effect of vast magical forces working beneath it. Almost like a magical conspiracy story.
Best scene in story:
When Duffy crosses the alps and the beasts keep him safe through the misty terrain, it's uncanny and very magical and sets up the scope of the forces at work quite nicely.
Opinion about the main character:
Duffy is a good bloke, but a bit hard to get a grip one since he spends a good chunk of the book either as Arthur or pushed about by Aurelius and other forces greater than himself.