Missing the Point

All the work I've done so far for my first Blockbuster novel fits on half a page of unlined paper in the form of a web diagram (The best thing I learned in 4th grade). I've come up with a lot of scenes and even outlined the core themes I would like to address. What I don't have is a reason to write the story in the first place.

According to Randy Ingermanson's Snowflake Method (which is loosely "based" on fractals), the first step is to

...take an hour and write a one-sentence summary of your story. Something like this: "A rogue physicist travels back in time to kill the apostle Paul." (This is the summary for my first novel, Transgression.) The sentence will serve you forever as a ten-second selling tool. This is the big picture, the analog of that big starting triangle in the snowflake picture.

So while I feel like I have the take away message ("think like me, muahahah"), I lack the crucial part that would complete the following dialogue...

Genuinely Curious Guy: "Hey reader of obscure novice literature, whatcha reading?"
Semi-literate Fan: "Shepley's new Blockbuster Thriller!"
Genuinely Curious (soon to be disappointed) Guy: "Oh, yeah! What's it about?"
Semi-literate Fan: "Oh, its about..." *insert Snowflake step 1 here.

Right now I have, "A vacationing professor discovers the secret behind Rio's darkest favela." Needs work... maybe it will receive some.