This viral ad about high school love has a frightening twist you won't see coming.

Every now and then, we’re reminded of the remarkable power of advertising. We’re shown that while the majority of ads sell us products or ideals or people, sometimes they’re capable of eliciting an emotional reaction that really teaches us something.

This is one of those times.

A recent ad from BBDO New York appears to tell the story of a very sweet and tender high school romance, but when viewers’ eyes are opened to a much deeper mystery, we’re faced with a disturbing reality.

The production begins with a student named Evan, who starts etching words into a table in the library. When someone writes back to him, we become enthralled in the exchange between this innocent high school boy and his anonymous friend.

Sweet music plays in the background and we’re captivated by Evan going about his school day. A teen simply trying to find out who is the mystery person writing back to him.

That's until a student enters the hall with a gun and everyone starts screaming.

The ad then tells us that while we were watching Evan's love story unfold, another student was planning a shooting. What's truly alarming is that you can see it - as the filmmakers take you back through Evan's love story. 

In the background of each shot is another student exhibiting clear signs of frustration and plans for violence - but we were wholly distracted by what was happening in the foreground.

The advertiser, Sandy Hook Promise, go on to say that gun violence is preventable if you know the signs.

With almost three million views on Youtube alone, the ad has struck a chord with people all over the world.

Powerful and real - if you discuss one thing with your family tonight, make it this https://t.co/oTelgzy7Mn #ProtectOurKids @sandyhook

— GaGirrl (@GaGirrl) December 5, 2016

This is one of the most effective PSAs I've ever seen. Wow. https://t.co/2bvECO4FEa

— Paul (PJ) Rieckhoff (@PaulRieckhoff) December 5, 2016

Whether or not the signs for gun violence are a) clearly discernible or b) really do give us a chance to intervene is contentious - but the ad does demonstrate one crucial message: We see what we want to see. And that is really, really scary.