Hmm...Digimon? Can't say I've ever understood that show...watched a bit, got annoyed by the fact that nothing seemed to make any sense whatsoever and stopped watching. Besides, the voiceovers sucked.
Ahem, anyway...there would have had to be quite some time since the pok?mon and humans were separated if we are to be able to talk about elders. So how about this? Both our pok?mon and the humans (Ourselves or whatever characters we feel like playing) feel that there is clearly something wrong with the world but both lack the power to fix it. However, there are tales, in both worlds, of a time when the two factions lived together. Most people consider them simply that, superstition and fiction, but obviously our characters consider it worth a shot. As does a certain human organisation, one which knows for a fact that pok?mon existed and finds it fairly easily deducted that the tales of their power are also true. (They would have found one caught permanently frozen into the wall of a cave or something like that) Our (human) heroes find out about this as well (Perhaps this organisation is after something that they posses? Or maybe the gate could only be opened by those with pure souls (It's a bit of a clich? but it works), which is why they would need the 'cooperation' of the people themselves?
The pok?mon, on the other hand, have always been taught that humans are greedy and narrow-minded and that they have been sealed away because of the destruction they bring (Which is fairly logical considering what happened) but our (Pok?mon) heroes believe that things aren't all that simple.
In addition, both worlds face their respective treaths. The pok?mon world has the shadow pok?mon and the human world has this big, evil, corporation-thingie messing it up, and both have to admit that they need each other's help and decide to take the chance.