The "Incomplete Protein" Myth
All plants are a complete protein source.
First, quick science lesson. There are 20 amino acids that are the building blocks for forming thousands of proteins necessary for human life. Nine of them are referred to as “essential,” as our bodies cannot make them. Thus, we must obtain those from our food.
Made it? Good job.
Onward . . .
Debunking the Myth: Incomplete Protein vs. Complete Protein
In recent decades, there has been a myth widely circulating, stating that there are such things as “incomplete proteins” and “complete proteins.” As you might guess, the so-called “complete” proteins have been seen as better than or preferable to the “incomplete” variety.
Meat, soy, and quinoa, for instance, have been praised as “complete” while beans, corn, rice, and most other plants have been deemed “incomplete.”
However, this bifurcation is a myth.
The fact to bust this myth is this: All plants have all nine essential amino acids.
As such, how could there be an “incomplete” protein?
As long as you’re not subsisting on just one food, odds are that you’ll be just fine in getting the variety of essential amino acids (EAAs) that your body needs. While plants vary in their levels of these nine EAAs—e.g., one may have more leucine while another has more tryptophan or valine, etc.—eating a diversity of plants will ensure that you get what you need. Again, this is really only a concern if you’re subsisting on only one to a few plants (say, eating only rice, all the time).
But what about the “limiting amino acid,” meaning the EAA that is present in the lowest amount in that given food? Any particular plant may have a particular EAA that may be quite lower than the other eight EEAs, but, again, as long as we are consuming a variety of plants, we will be getting higher levels of that particular amino acid in other foods, filling in the gap. As humans, we eat meals. We combine foods. And even if you find yourself having a relatively simple meal from time to time, your other meals throughout the day and the week will likely quite easily bring in the needed diversity.
Nutritionist Simon Hill states, “The problem with this narrow view of protein consumption is that, instead of looking at the overall dietary pattern, it judges a single food in isolation.” That's just not how we eat.
What about food combining?
What about “complementary proteins”? Do I need to make sure I’m eating rice and beans at the same meal?
You certainly can, and it’s great when it happens, but don’t worry or stress about it. There is no need to be concerned about this, especially if you are eating a diverse variety of plants. Simon Hill also addresses this question, noting, “Your body has a constant pool of amino acids from the food you consume across the day . . . . [Y]our body will get all the EAA protein building blocks it needs.” Not only do our bodies have lots of building blocks to work with from the food we consume in a given day, but they also maintain “pools of free amino acids” so they can do all the combining we need on our behalf. Dr. Michael Greger addresses this and more in the video linked below. 
Don’t shy away from a dietary pattern rich in plants for fear that you won’t be getting enough protein.
Finally, if you’d like to dive into this a bit more, I highly recommend this 4-minute video that explains a bit of the history of this myth and why it is false: “The Protein-Combining Myth.”
P.S. If you’re curious if those eating plant-predominant and plant-exclusive diets get enough protein, rest assured. In all likelihood, with a diversity of plants, they are. See this video from Dr. Michael Greger on NutritionFacts.org for more on this.
 Simon Hill, The Proof is in the Plants, 279.
 Ibid., 280.
 Dr. Michael Greger, NutritionFacts.org, “The Protein-Combining Myth.”