Last issue, Mephisto kidnapped The Invisible Woman and said only an “unknown quantity” could defeat him and get her back. Reed Richards figures that would be X-Factor, so he seeks their help.
Meanwhile, Mephisto talks to himself at length and explains that he really didn’t want to get revenge on Franklin Richards (which we knew because he traded the kid for his mom), and he really didn’t want Susan Richards’ pure soul (which we knew because he told her that her soul wasn’t so pure after all), what he really wants is a “noble” soul to torment.
So for this issue Mephisto torments the members of X-Factor just like he did the members of the Fantastic Four last issue, and then trades Susan Richards for Jean Grey.
I guess he prefers redheads.