The series focuses on the supporting cast of the X-Men, whose ranks have been severely depleted following Decimation.
In House of M #7, the Scarlet Witch removes the powers of the overwhelming majority of the world's mutants in an event, eventually called "M-Day".
X-Men: The 198 revolves around the tension and conflicts of the 198 mutant refugees on the Xavier Institute grounds and the Sentinel Squad ONE.
These include the following: The list excludes Xavier Institute staff members and the remaining student body as well as the Sentinel Squad O*N*E. The 198 Files is a database maintained by O*N*E that attempts to classify and provide intelligence on the mutants whose powers remained after M-Day.
The one-shot X-Men: The 198 Files is set in the style of a government dossier and sent to O*N*E's deputy director Valerie Cooper.