Ultimate Fantastic Four #21
The story's called "Crossover" and when we're talking about the Ultimate Fantastic Four, you know it's not just across the street!
Superstar scribe Mark Millar returns to Ultimate Fantastic Four, and he's brought white-hot X-Men: Phoenix Endsong superstar Greg Land with him! Even in their short careers, the Ultimate FF have seen a lot of amazing things - but nothing will prepare them for the world they're about to enter! Reed Richards has used his scientific genius to contact an Earth in a surprisingly familiar parallel dimension - and he's ready to visit!
Is this dimension indeed our own, familiar (if arbitrarily-named) 616 Marvel Universe? Well you won't get any clues here, but you can check out these awesome preview pages featuring the Thing punching out a Tyrannosaurus.