It's basically the Sliding Doors-esque idea that the smallest thing can have the biggest impact on your life - in this case, choosing to sack off going round your Mum's for lunch means you find yourself testing your fitness as you race around New York with a phone you found, avoiding the shots of a gunman. (Karma's a bitch.)
I think this is a film that needs another watch. And I can't decide what I'd think about it if the leads weren't pretty. But being the geek I am, I researched the film after and found that all the dialogue was improvised. Impressive stuff. Plus there's a lot of background on the dog that features in the film. And I love dogs.
Watch the film, then indulge your inner film buff here.