8 months after Summer Knight Harry is asked to to be on a talk show. While taping The Larry Fowler show each of the guests is on the show to actually meet Harry: Mortimer Linquist, a local medium, tells Harry that his girlfriend Susan is still alive and is somewhere in Peru. Father Vincent, a Vatican priest, wants to hire Harry to find the shroud of turin. A university professor by the name of Ortega is a Red Court Vampire and challenges Harry to a duel to simplify the battle and to make an offer to have Chicago be a neutral zone in the war with the courts.
On top of all that the police detective in charge of the strange and unknown, officer Murphy, hires Harry to to investigate a man who has died of every disease all at once.
Outside, Harry is attacked by the Denarian Ursiel, a fallen angel attached to a mortal host. Michael Carpenter and two other Knights of the Cross, Shiro and Sanya, rescue him and ask him to drop his case, but Dresden refuses.
In his Lab, Dresden consults Bob his oracle skull. He learns the knights received an angelic prophecy: if Dresden is involved, he will die. However, they did not receive the whole prophecy, which states that if Dresden is not involved, all the Knights will die, as will everyone in the city of Chicago.
The Archive, a little girl that Harry names Ivy contains the sum of humanity's written knowledge, and her bodyguard Kincaid, arrive to make the arrangements for Ortega's duel. A neutral mediator selected by the White Council, she is the guarantor that the duel is conducted by the Accords.
Harry Dresden tracks the Shroud to a boat and is captured and handcuffed by the thieves while trying to recover it. Deirdre, another Denarian, attacks the boat and kills one of the thieves. Dresden fools the Denarian into taking a decoy safe that does not contain the Shroud and leaving. To save her life, the surviving thief, Anna Valmont, steals Dresden's black leather duster and flees with the Shroud. Valmont returns a short time later to un-cuff Dresden and he is able to escape.
Since Michael is out of town, Shiro volunteers to be Dresden's second in the duel with Ortega, whose second is Thomas Raith. They meet at McAnally's Tavern to settle the details of the duel: the weapon is "will", at sundown the next day, at Wrigley Field.
Susan, who has been in touch, is waiting outside McAnally's with a tux and a limo. She has tickets to a high society art sales charity event run by Johnny Marcone, where the Shroud will likely be sold. Marcone attempts to evict them, but they evade capture and locate Anna. The sale is interrupted by the Denarians, who seize the Shroud and kidnap Dresden.
Nicodemus, leader of the Denarians and father of Deirdre, pressures Harry to join with the Denarian Lasciel or die. Dresden refuses. Before Nicodemus can kill him, Shiro arrives and trades himself for Dresden for 24 hours. Dresden is almost re-captured, but Susan, enhanced in battle from her semi-vampire status, fights Deirdre to a standstill allows him to escape. Susan and Dresden are pursued back to Dresden's apartment, where he activates the powerful defenses. However, this means Susan cannot leave until dawn. She is nearly driven mad by the scent of the blood, dripping from his wounds. To save his life and hers, Dresden magically binds Susan and has sex with her in order to quell her hunger. (She finds the hungers similar.) She reveals that The Fellowship of St. Giles is an organization of half-turned humans. They helped Susan understand and control her new, semi-vampiric nature; and now, she's helping them exterminate Red Court vampires in South America. Martin is her Fellowship mentor, not her boyfriend. Her commitment to The Fellowship is the reason she is abandoning her old life in Chicago.
In the morning, Susan and Harry leave the boarding house and seek out the Knights. They discover that Father Forthill has an Eye of Thoth tattoo similar to the one on the unidentified corpse in the morgue, which he and Vincent received. Harry deduces that Vincent is a phony, the Knights defeat Saluriel posing as Vincent, and Harry forces him to reveal that Nicodemus' plan is to create a deadly plague curse, powered by the Shroud. The spell will be cast that evening at the airport. As an international travel hub, O'Hare Airport is an excellent place to disseminate their plague.
With Susan as his new second, Dresden heads to Wrigley Field to fight his duel with Ortega. The Archive brings out a piece of mordite, a stone of anti-life. It is enchanted to move by willpower, the chosen duelling weapon. If it touches either duellist, or either camp cheats, they die. Being overpowered, Ortega draws a weapon but is shot by Martin. A swarm of Red Court vampires surge onto the ball field, attacking everyone and covering Ortega's escape. Dresden, Kincaid, Susan and The Archive defeat the vampires. The Archive asks Kincaid who cheated first. Kincaid said he didn't see it, but that Dresden was winning when the shots were fired. Dresden wins the duel by default.
Dresden races to the airport with the Knights to save Shiro and stop the plague curse, finding him severely tortured and near death. Shiro entrusts Fidelacchius to Dresden, telling him to pass it on to the right person. Shiro reveals, dying, that Nicodemus is going to St. Louis by train to spread the plague curse, which can be stopped by taking the Shroud from Nicodemus.
Dresden enlists the aid of Marcone and Ms. Gard to catch up to the St. Louis train. Dresden, Marcone, and the Knights battle the Denarians to retrieve the Shroud. Thanks to a hint from the dying Shiro, Dresden finds Nicodemus' weakness (his noose) and takes the Shroud, cancelling the plague, but fails to kill him. Dresden and Michael almost die while escaping from the train, but Marcone rescues them.
Dresden recuperates in Michael's home. He receives a two-week old letter from Shiro. He had been diagnosed with cancer and came to Chicago knowing he would sacrifice himself to save Dresden. This news comforts Michael and Sanya, making Shiro's actions into the deliberate acts of a courageous man.
The next day, Ebenezar calls and tells Dresden to watch the news. In a freak accident, an old Soviet satellite, Kosmos 5, crashed into Casaverde, Honduras—Ortega's secret fortress. There are no survivors. Dresden realizes that McCoy has deliberately killed Ortega with the satellite.
Dresden trails Marcone to a secluded, rural hospital. He discovers that Marcone had the Shroud stolen hoping to cure a comatose girl. Dresden tells Marcone it will take three days to see if the Shroud will heal the girl. After that, the Shroud must be returned to the Church. Marcone agrees.
After the Shroud is anonymously returned to Father Forthill at St Mary's, Dresden goes to Michael's house for Sanya's bon voyage barbecue. Nicodemius drives by, tossing a coin into the yard. Michael's youngest son Harry is about to pick it up, when Dresden snatches it just in time to prevent the child from becoming a Denarian. Harry rushes home to bury Lasciel's coin in his basement lab.