Crusade

Several years after the end of the Great War (detailed in the main Babylon 5 TV series) the former servants of the now-exiled Shadows attempt to avenge their old masters by introducing a powerful biological weapon into Earth's atmosphere. After five years of adapting itself to humanity's genetic makeup, it will kill every human being on the planet. Since this plague was the product of a technology far ahead of humanity's, there isn't enough time to develop an original solution - instead, humanity will have to comb through the ruins of older alien civilizations with the hope of finding some ancient, advanced technology that can cure the disease. Leading this high-stakes archeological mission is a starship of the new Interstellar Alliance, the Excalibur, and its crew drawn from the Alliance's elite troubleshooting corps, the Rangers.

At the end of the television movie Babylon 5: A Call to Arms, the Drakh fleet was defeated but "poisoned the well" by dumping a plague into Earth's atmosphere. Calculations say that all life will be dead on the planet within five years; lives are already being wiped out across the globe. Earthforce, operating out of Mars during the crisis, has chosen Captain Matthew Gideon to lead a team consisting of Earthforce scientists, archaeologists, a telepath, a thief and a techno-mage in order to find a cure while working with the Rangers and gaining assistance from the crew manning space station Babylon 5.