from the back cover:
What if the Fantastic Four were never born? How many other heroes whould be affected--who would die? Who would not decide to become a hero? Who would switch sides, and become a villain?
This is what Kang wants to happen. He and his alternate-reality duplicates have targeted the early 1960's as the time-space to begin their (his?) domination of the planet Earth. The plan: to eliminate the heroes of the 20th century, to make life easier for Kang. Lots easier.
Can your supergroup foil this nefarious plot? Can you make it through the myriad of timestreams, finding the right Kang at the right time? Have you ever operated a time machine in any of your lives?