Seer, a minion of Doctor Doom, has a prophecy of an all-powerful being known as Onslaught -- no surprise that megalomaniacal Doom seeks to harness its power!
Within Castle Doom in Latveria, Dr. Doom has ordered his seer to bring forth glimpses of the future to aid him in his plans of world conquest. The seer manifests a vision of the battle against Onslaught which is to take place many years later. Doom demands more, but the exhausted mystic collapses, much to the tyrant's frustration.
Something about Onslaught's appearance seems familiar to Doom, so he pulls up on his video monitors film footage of recent incidents. He finds what he seeks: the initial conflict between Magneto and the X-Men that first brought the existence of super-powered mutants to the public's attention.
The next morning, determined to learn if the link between Magneto and Onslaught goes beyond their similar appearances, Doom intends to use his time platform to travel to critical points of Magneto's life to observe, while remaining shielded and cloaked aboard his machine. Bringing the seer along, he first ventures to the recent past and Magneto's attempted takeover of Cape Citadel. Doom is impressed by the abilities of both the master of magnetism and the mutant youngsters assembled to oppose him. As Magneto makes his escape, Doom orders the seer to establish a psionic link with him in order to locate Magneto in the timestream.
They then travel farther into the future, when the X-Men defeat Magneto in the Savage Land, and then farther still to another battle within an underground base in Antarctica. This time, the master of magnetism faces the second generation of X-Men, the conflict having damaged the systems that are in place to protect the base from the surrounding volcanic activity. While the conflict rages, the time travelers' presence catches the attention of Phoenix, thanks to her telepathic mastery. Using her astral form, she confronts Doom, and demands to know why he is there and whether his platform may provide a means of escape. When Doom refuses to divulge any information, she psionically damages the time platform controls within his gauntlet.
The damage throws the machine off course, and Doom and the seer now find themselves in the devastated New York City of an alternate future when the Sentinels have taken over. They are approached by a Sentinel which orders them to surrender, but Doom easily disables it. He lands the platform, hiding it in a safe spot and leaves it with the seer while he departs to look for supplies to make repairs. Electronically cloaked, he follows the older Wolverine, Colossus and Storm as they are about to make their ill-fated assault on the Sentinel command center. Doom gathers his supplies as the robots destroy the X-Men one by one, then he leaves the facility.
As Doom makes repairs, the seer tells him he detects no trace of Magneto, surmising he has probably died. His work completed, Doom sets a course for the past, but the seer experiences a psychic backlash when they travel past the moment when Onslaught would be at the peak of his powers. The seer tells Doom that during that one instant, he felt the presence of another mind within the entity, that of Charles Xavier. At that, Doom decides to travel to and observe pivotal points in Xavier's life. They witness his reaction to the death of Thunderbird, his attempt to shut down the Dark Phoenix, and his near possession by a Brood Queen.
Their final view of Xavier is when the professor would be critically wounded by the Strucker siblings during Magneto's trial. They watch as Magneto agrees to Xavier's wishes for him to lead the X-Men and mentor Xavier's students in his absence. Doom looks upon the scene with disdain, believing Magneto would not fulfill his vow, and is merely comforting a dying man, or more likely, conniving to destroy the X-Men from within.
Doom then opts to confront Magneto some time after the defeat of Onslaught. He orders his seer to pull the truth from his quarry's mind as they battle. Magneto warns Doom that to seek Onslaught's power is folly, but once the seer, in his astral form, succeeds in extracting the information, the tyrant departs. The information leads them to Avalon where and when Xavier's psychic assault on Magneto leaves the latter in a coma, but also births Onslaught. With the aid of the seer, Doom observes the struggle on the psychic plane, finding the dark seed that would become Onslaught.
Using an absorption device, Dr. Doom tries to capture the entity, but it discovers him and turns to attack. Upon contact with the time platform's shield, a massive energy surge erupts, sending the time machine back to its point of origin. Although he's failed in his goal, Doom contents himself with his acquisition of valuable knowledge. Unwilling to share this knowledge with anyone, his disintegrates the seer.
Note: Dr. Doom appears here between his appearances in AMAZING SPIDER-MAN #5 and FANTASTIC FOUR #23. The X-Men, Professor X and Magneto appear here in scenes parallel to those in (UNCANNY) X-MEN Volume 1, #s 1, 63, 94, 113, 136, 142, 161 and 200.