Nancy Adams (Blake Lively) was so shaken by her mother’s (Janelle Bailey) untimely death that she dropped out of med school. In an attempt to feel closer to her late mother, she decided to go to the same Mexican retreat where her mother told her she had been conceived. An avid surfer, Nancy plans to search for her mother’s favorite stretch of beach.

When she arrives, Nancy is so impatient to find that idyllic spot that she impulsively heads for the ocean with her surfboard, handbag, and smartphone, leaving her tired companion at the hotel. She gets a ride to the shore from Carlos (Oscar Jaenada), who is happy to serve as Nancy’s chauffeur and navigator. After depositing her at the secluded cove, he drives away. Nancy is not worried about being left alone, since she does have cell phone service. So she blissfully paddles out to deep water on her surfboard where she’s surrounded by a pod of playful dolphins as she starts riding the waves.

Things change when she spots the carcass of a humpback whale. What Nancy doesn’t realize, until it’s too late, is that she’s in the feeding ground of a shark.

She receives a nasty gash from the initial attack of the shark but is able to swim to a tiny nearby island. Her medical training comes in handy as she quickly fashions a tourniquet from part of her outfit.

However, with high tide coming in a matter of hours, she knows that she’s got to get to the beach before her temporary sanctuary is overrun by the rising sea level. The shore is 200 yards away, which is too far to swim with a determined predator steadily circling as her blood drips into the water.

Two potential rescuers (Jose Manuel Trujillo Salas and Angelo Josue Lozano Corzo) show up, but hope fades when they start swimming without noticing that Nancy is in trouble. The next beach goer (Diego Espejel) does see that Nancy needs help, and takes advantage of her predicament to steal her phone and other personal effects that were left on the sand.

So Nancy must survive by her wits, a daunting challenge given her situation. Thus unfolds The Shallows, an engaging thriller expertly directed by Jaume Collett-Serra (Non-Stop).

The movie borrows elements from Jaws, Castaway, Blue Crush, and MacGyver. The good news is that it all has been sewn together quite seamlessly into a movie that will keep you on the edge of your seat.

Excellent (****). Rated PG-13 for bloody images, intense scenes of peril, and brief profanity. Running time: 87 minutes. Distributor: Columbia Pictures.