English actress Catherine Steadman, most recently seen in the terrific series “Downton Abbey,” has written her first novel --- and quite a memorable debut it is. Smartly released in the summertime, SOMETHING IN THE WATER is a potboiler of a psychological thriller that spins further and further out of control, all the while making it an ideal beach read.

If you're a fan of Tarantino films, or if you think Deadpool 2 is a wild comedy, then you may be the type of person who will get a nice thrill over the novel’s opening line: “Have you ever wondered how long it takes to dig a grave?” That thought belongs to our narrator, Erin, on October 1st of the current year. The story then jumps back a few months prior to July 8th, at which time we find young Erin and Mark celebrating the anniversary of the day they first met.

What follows is their anticipated engagement. However, before they head off on their honeymoon, Erin has to finish taped interviews on the biggest project of her young filmmaking career. She is putting together a documentary focusing on three different prison inmates and following their various situations and outcomes. The final prisoner is a purported crime lord named Eddie Bishop. Erin is waiting patiently for a phone call on July 11th from one of the last remaining East End gangsters. His participation in the project will really put things over the top for Erin's success. However, little does she know at that time where her relationship with Bishop will go.

September 4th comes around quickly, right at the professional height for both Erin's filmmaking career and Mark's job as a top investment banker. They head off on their honeymoon to a tropical oasis in Bora Bora. They are living a turquoise, warm-sanded dream until the moment their lives are changed forever. One day while scuba diving, Erin and Mark come upon a wreck under the water, where papers are found floating, but that is only a precursor of things to come. Mark fishes out of the water a black canvas duffel bag fastened by a padlock.

"SOMETHING IN THE WATER makes for a wild, rollercoaster ride of a thriller with a conclusion that cleverly resolves things."

The two bring the bag onto the boat they chartered and agree that they will just turn it over to the hotel staff when they return to land. When Erin comes out of their bathroom the following day, she sees that someone has left the bag, still padlocked, at the foot of their bed. Mark finds the situation humorous and thinks nothing of it as they head out for a day of hiking with another young couple they have met up with on their honeymoon. They return to find the bag still waiting for them on the bed when Erin decides to take a scissor to the padlock and open it up.

Erin soon regrets opening the bag shortly after the contents are revealed. In separate vacuum-sealed baggies, they find packs of American money in bundles each labeled $10,000, an iPad with some other pieces floating loosely, and what appears to be about a million pounds in diamonds. Erin and Mark realize that they may have hit the jackpot and are dealing with mixed emotions surrounding the fact that whoever lost this bag of goods most definitely would be looking for it.

They take a risk in packing the items for their flight home. Erin makes the mistake of turning on the burner cell phone that was also found with the other assorted treasures and is horrified to be met with a message reading “Who is this?” The phone number registers as Russian in origin, and Erin realizes the flotsam and jetsam they took from the water is going to bring them a lot of trouble. Things get extremely serious when they learn that the British couple with whom they took the hike has turned up dead. It seems that the porter mistook the couple for the one who attempted to turn in a “bag” found in the water, and those who were inquiring about it promptly eliminated them.

Erin and Mark are soon back in their new London home making plans on what to do with their newfound wealth. Mark claims to have all these connections through his business that can help them sell off the diamonds and place the receipts in a Swiss Bank account. Erin finds Mark acting very erratically and starts to wonder how well she really knows the man she just married. Steadman nicely milks every moment of suspense and paranoia, and places the reader right inside Erin's mind as she goes through the spiral that this unexpected bag has set her on.

Erin believes that she and their home may be under surveillance from a group called SO15, a Special Counter Terrorism Command. As she begins to become more and more suspicious of her new husband, Erin reaches out to Bishop for advice from the criminal perspective. It turns out that the most important part of the goods they found was the iPad, which had an external thumb drive that contained an encrypted spreadsheet with very valuable, highly illegal information --- information for which someone will continue to kill in order to obtain it once again.

This brings us full circle back to the opening of the novel, which has Erin pondering how to dig a grave. I will leave things right there as SOMETHING IN THE WATER makes for a wild, rollercoaster ride of a thriller with a conclusion that cleverly resolves things. I think it is safe to say that Steadman may find her literary career soon surpassing her acting career if she continues to produce sharp, intelligent thrillers like this one.