Plain is the opposite of Complicated. Complicated is similar to Complex. Complex is related to Theory. Theory is related to Gravity. 
House is related to Neighborhood. Neighborhood is related to Colony. Colony is related to Numerous. 
Plain <--Antonym--> Complicated <--SimilarTo--> Complex <--RelatedTo--> Theory <--RelatedTo--> Gravity 
House <--RelatedTo--> Neighborhood <--RelatedTo--> Colony <--RelatedTo--> Numerous 