Mole Man's monster Giganto interrupts the Fantastic Four's day off from work. The superheroes spring into action to stop the powerful creature from destroying the city and, more specifically, the Baxter Building.
Johnny ends up traveling to another dimension while leading his friend Frankie on an unauthorized tour of Reed's lab. Meanwhile, Reed, Ben and Susan are left to contend with a seemingly harmless insect.
The team has to stop Dr. Doom from destroying the city, while trying to figure out whether Reed betrayed them by purposely exposing them all to the cosmic rays that gave them their super powers or if Doom was behind that as well.