For your players
Use other solutions
Intelligence is only one of 3 mental stats, and the one that refers mostly to book-learning, puzzling, connection-making and thinking stuff through. However, lacking it doesn't make you a bumbling fool; it makes you someone who solves problems in other ways.
For example, if it's crucial that the characters know who this guy is or where he's from, they could just, you know, ask him nicely (using Charisma) Or, if they have a hunch that he won't appreciate that (using Wisdom) they can always ask someone else.
They should probably try to make friends with some Sages or Oracles and the local librarian or Wizard's Guild. Whenever critical information is needed; go find someone who knows and let them help you solve the problem. The party doesn't have to do everything on its own, especially when it comes to raw knowledge.
For you as a DM
Learning without intelligence
There's also a lot of information you can learn without knowing the facts. In your example, I might not know who the person in plate is, but I can probably identify:
- whether he is a capable warrior or wearing plate for show (based on how he wears it)
- whether he is a local authority, an adventurer, or an invader (based on the reactions of the crowd)
- whether he is affaliated to some religious entity (based on obvious symbols)
- whether they are here looking for me (based on how they scan the room and maybe on how suddenly people around me are taking their distance)
and probably a bunch more. I might not know his name, his rank, or his order, but I can probably figure out (some of) his reason for showing up, his capabilities and his standing with the local crowd if I have high Wisdom or Charisma and associated skills.
(While on the other hand someone with very high Intelligence but low Wisdom/Charisma might know his name, family, rank, order, history and still be completely oblivious that he showed up to arrest him until he feels a strong hand on his shoulder.)
I don't think a group of characters of low intelligence are neccesarily problematic, even in stories with complex stories and scheming. They just need to change the way they approach the problem and you need to change the way you describe problems.
Things intelligent characters are good at (and thus your player's characters aren't) include having factual knowledge, connecting the dots, and thinking far ahead. We put these things in normal descriptions, but for this campaign you'll probably need to take them out (unless shared by an intelligent NPC).
Instead, only include what can be immediately visualised from the surroundings, with as little prejudice as possible. But increase the description of what can be seen clearly a bit more; the characters will be making up for lack of background knowledge with more direct information. And keep in mind the difference between knowing what something stands for and that is stands for something; include a lot of the latter.
As you will see, both descriptions below give players enough information to handle it and have some pointers that something might be going on, but the former has more and the latter will require the players to get someone involved who can share more information with them. For example; both stress that this is a well organised force of goblins/orcs, but the first gives much more leads than the second.
Example description for regular groups
"Following the warcries, you come upon a battleground. 3 merchant's carts stand in a semi-circle, 1 is on fire. A group of humans in light armor and with spears are holding a defensive line against a combined group of Goblins and Orcs wielding high quality weapons for such creatures and carrying banners and symbols of Gruumsh on their shields. You do not know what is causing these two groups of creatures to fight together, as they are usually on bad terms. The humans seem to be slowly losing the battle, and corpses strewn around the field show that it's being going on for some time. You don't see any non-combatatants with the merchants and they seem to be heavily guarded, especially for a caravan travelling through what was considered 'safe passage' by the locals."
Example description, same encounter, minus the intelligence
"Following the warcries, you come upon a battleground. 3 carts stand in a semi-circle, 1 is on fire. A group of humans in light armor and with spears are holding a strong defensive line against a combined group of well-trained and equipped Goblins and Orcs. They all carry the same profane symbol on their shields. The once strong human force seems to be slowly losing the battle. A number of corpses from both sides lie around the battlefield."
- Your characters should ask NPCs for help when they need it (and they will)
- Everyone should try to rely less on facts and connections and instead act more "in the moment"
- Emphasise the things someone without book-learning and strong reasoning skills can still pick up from his environment; low intelligence does not make you a bumbling idiot, it just makes you someone who doesn't rely on structured, learned knowledge to make decisions.