If you've been able to negotiate the current season of "Lost," with its many alternative realities, you should have no trouble with "Uncertainty."

On a July 4 morning, Bobby and Kate (Joseph Gordon-Levitt, Lynn Collins) stand in the middle of the Brooklyn Bridge debating how to spend their day. They can head east into Brooklyn and attend a barbecue being thrown by her parents. Or, they can go west into Manhattan and see what adventures await.

In Scott McGehee and David Siegel's fantasy they do both simultaneously; the two stories are entwined throughout. We can tell which is which by the clothes Bobby and Kate wear in each; in fact, the two realities are called "Green" and "Yellow."

The "green" Brooklyn story line is purely domestic. The unmarried couple attend a family reunion. Kate goes head to head with her mother over the things mothers and daughters usually argue about. An uncle - a former boxer - has early onset Alzheimer's. A sibling (Olivia Thirlby) seeks advice from her big sis. Bobby and Kate pick up a dog wandering in the street.

Hanging over it all is the couple's secret: Kate is pregnant and hasn't decided whether to keep the baby.

Real people. Everyday situations.

The other story line is something else.

In the back seat of a cab, Kate and Bobby find a fancy new cell phone. Hoping to return it, they call one of the numbers listed. Setting up a rendezvous with the "owner," they witness a murder. Then they're off running, an Asian gunman in furious pursuit. They slow down long enough to concoct a $500,000 ransom demand for the return of the phone.

So what's on the phone? Corporate espionage? Government secrets?

Doesn't matter. McGehee and Siegel (they made a terrific debut a few years back with the Tilda Swinton thriller "The Deep End") are less concerned with conventional melodrama than with seeing how two individuals will behave in wildly different situations.

Go east or go west ... either choice may end up dictating how you'll live the rest of your life. The implication is that under certain conditions, any of us can be capable of anything. Each of us can be several different persons.

"Uncertainty" is very well made, but it works mostly because of the two lead performances. Gordon-Levitt ("(500) Days of Summer") has been all over indie film for the last couple of years, and the role of Bobby draws heavily on his likability and innate moral authority.

Collins, known mostly as Hugh Jackman's girlfriend in the "Wolverine" solo project, scores as a young woman facing several big choices and unsure of what to do.

'Uncertainty'

DVD, $24.98

(MPI Home Video)