The next book in the Sigma Force series, The Doomsday Key, which Rollins published in June 2009, follows the SIGMA team as they investigate three murders on three different continents with similar circumstances. The culprit is a Norwegian corporation with ties to the Club of Rome. The leader of this corporation is trying to stop overpopulation and loss of food by any means necessary. Like the other novels in the SIGMA series, there is a fantastical element to this story. The team travels to the Coliseum in Rome, the Lake District of England, Bardsey Island, and a prison in France. It's ultimately revealed that what the characters are dealing with is an ancient fungal parasite that was discovered by the Egyptians and used by them and the Druids as a bioweapon. Mummies found in a peat bog contained the fungus and it was used to try to sterilize crops to help stop overpopulation, but it mutated back to its deadly form. Ultimately an ancient embalming compound is revealed to be the cure. A surprise twist at the end indicates that perhaps the SIGMA team had been traveling with one of the Guild's mysterious leaders before his death at the end of the novel, a fact that not even the Guild members hunting them knew.