Set in the aftermath of the House of M event, where the Scarlet Witch depowered the majority of the mutant population on "M-Day," this issue explores the lives of the remaining 198 mutants. The Xavier Institute becomes a sanctuary, now under the watchful eye of Sentinel Squad ONE, a government initiative aimed at monitoring mutant activity. The story delves into the tensions and challenges faced by these survivors in a world that has drastically changed overnight.

The narrative opens with Magma (Amara Aquilla) on vacation in Guadeloupe, where she meets and falls in love with a fellow mutant named Antonio. Their romance is tragically cut short when M-Day strikes, stripping Antonio of his powers mid-flight over an active volcano, leading to his death. This personal loss underscores the broader theme of sudden vulnerability among mutants.

Meanwhile, in Mutant Town, Toad and Lorelei confront members of the anti-mutant group Purity, highlighting the societal tensions and prejudices that have intensified post-M-Day. The issue also introduces Mr. M (Absolon Mercator), a powerful mutant who arrives at the Xavier Institute, hinting at future developments in the series.

X-Men: The 198 is a five-issue limited series that continues to explore the ramifications of M-Day on the mutant community. The series was later collected in the trade paperback Decimation: X-Men – The 198, released on August 30, 2006.