Doom travels back in time to investigate a connection he perceives between Magneto and Onslaught. He figures out that Professor X created Onslaught after Magneto’s post-telepathic suggestion, but is unable to capitalize on that knowledge…For now.
Although he witnesses a lot of X-events during his time-travels, I’m not tagging all those characters because they’re not really involved in the story–they’re basically like movies he is watching as he witnesses the past. He does, in the end, actually briefly meet Onslaught on the astral plane, so I have made an appropriate character tag for that.
In a lot of ways, this is just a “history of Onslaught” primer.