Where do you think the DNAliens will invade?
Well honestly, I have never finished a story before, so all this advice is coming from someone with zero experience, but here is what I think: I don't think they should first appear in the Great Valley. Then the chaos will start too quickly before more of the story is established. It might be too much to see running and screaming before the reader has too much of an idea of what's happening. Now of course, if someone starts with turmoil, soon there should be a lengthy flashback to explain how it started. That technique lets readers know that there is trouble going on at that moment without the style of battle sequence description using narration which gets very old if used too frequently. Starting that way can add more excitement because they don't have to wait for a very interesting part, and then writing about how the past led up to it can make the reader have something to really look forward to. This is an interesting way to go about it, but it can be more difficult, so is probably done best by experienced fanfiction writers like some people I know here. I'm not saying that you're not experienced, I just haven't read a story of yours to even know if you write or not. So if it was a beginner like me, I would start slowly and work up to the awareness of this type of foe, especially considering the primitive inhabitants they'll meet. Dinosaurs seeing lasers, bombs, and spaceships will not promote very limited reactions.
Now that I've said that, let me answer your question. Since a Great Valley opening invasion might be too sudden, and since everywhere outside of that place seems to be called the Mysterious Beyond, if it were me, I would have them invade there. Also, since flying rocks have been seen from far away, if the DNAliens even landed close by, unless they have a cloaking device, I would recommend starting pretty far away. To ease into things, perhaps one or two scouts could enter the GV and start something small. Or to postpone chaos at all for a while, they could sneak in and not get detected, merely gathering information. Then they can return to the main group and plot a massive invasion. But hopefully that would give Ben, Gwen, and Kevin time to intervine the attack, or at least come quickly enough into it to avoid too much damage or any casualiites. Then as I had mentioned previously in this thread, perhaps there could be so many enemies that the trio needs help and the gang gets powers or some equipment for attacking and protection. Now if the DNAliens had a cloaking device and could land very close to the GV, then the invasion could happen more quickly, then the learning of the invasion by the trio and their method of getting their will have to be sped up by the writer so they have the time to help. As I said, starting chaos in the middle of the GV would seem to sudden to me, and in that case, the inhabitants would either fight back like Topsy, or would have the smarts to run like most everyone else, then the writer has the problem of the inhabitants meeting sharpteeth and finding a temporary home, unless the DNAliens capture them for whatever they are planning. If the dinosaurs get captured, perhaps the gang managed to escape or get out of sight, or the DNAlines had enough specimans or ran out of room to transport them. Then the gang and trio can team up stop them and get their families back. Now if the inhabitants flee, then when they're all gone, the trio could come to stop them and either A: not have innocent civilians to worry about, B: maybe the gang was curious and stayed to see the strange creatures or even get trapped inside the GV, in which case there is still the possibiblity of the gang helping to save the day, or C: something great could actually happen that would actually have to be thought of by someone who has anything to show from working in that area. In which case, try one of the fanfiction writers here for any assisstance. They have great ideas and experience to back them up. These are just ramblings of someone offering help in a field that he never succeeded in, so if you want to pursue this, try messaging a writer here that I'm currently familiar with like brekclub85, Explorer, The Chronicler, or someone else you will find in the fanfiction thread that I haven't had the pleasure of getting to know about their work yet. I hope that these thoughts of mine were vaguely interesting, to say the least. Later.
