Genetech and A.I.M. both want a specific energy source. The two groups hire out help to collect the source but only one person seems to be able to do so: Harness. By using a boy named Piecemeal, Harness does exactly what the name suggests and harnesses the energy sources across the globe by getting Piecemeal to "eat" them. The New Mutants (now X-Force) and the New Warriors are already on the trail of the energy sucking culprit when they run into each other, causing a quick scuffle. Deciding the work together, the teams split up and head to the biggest source of energy: Muir Island. The island mutants aren't too thrilled to get company (due to the influence of the Shadow King) but offer to help when they learn that the energy being gathered is to resurrect Moira's son, Proteus.
After capturing Harness and Piecemeal, the mutants try to extract the energy but to no avail. Proteus is reborn and fuses with the boy. The mutants don't have long to stop Proteus before he turns the earth into a huge, logical equation. They resort logic and reason as the only approach and offer Proteus the ultimate happiness: death.
All along, it seems as if the events had been manipulated from behind the scenes by Gideon and Toad. One man looks to control the economies of the world and the other simply wants to lead instead of follow.
Cannonball had seemingly joked about the name X-Force in the final issue of the New Mutants but the team is using the new name starting in this arc.
While the Muir Island mutants aren't technically X-Men, they were the only ones to appear in this arc (the real X-Men on some otherworldly adventure). Previous issues in countless books had speculated as to why Moira and her crew had been acting so strangely; the Shadow King is shown to have a grip on the mutants. This appears to answer those earlier questions (and comes to an apex in the Muir Island Saga).