Place the plum tomatoes, olive oil and lemon juice in a large mixing bowl. Add the couscous and mix thoroughly. Since there is no cooking involved, it is essential that there's an excessive amount of liquid in the bowl. The liquids will act as a 'cooking agent' for the couscous. The couscous will absorb all the liquids, puff up and become very flavorful. Add the red onion, red and yellow peppers, cherry tomatoes, and the basil to the bowl. Mix well. Season with salt and pepper, to taste. Let the mixture sit in the refrigerator for a few hours for all the flavors to amalgamate.