“Confessions on the 7:45” by Lisa Unger (Harlequin)

“Confessions on the 7:45” by Lisa Unger; Park Row (352 pages, $27.99)

The scenario is not uncommon: You sit next to a stranger on a plane or a train and, before you know it, you are confiding an intimate secret. It seems the perfect confessional as neither of you ever plans to — or hopes to — see the other again.

“Sometimes a stranger was the safest place in your life,” muses Selena Murphy.

“Confessions on the 7:45” succinctly emerges as a gripping story about marriage, parenting, families and revenge.

Barely making the 7:45 p.m. train home, Selena finds a seat next to a woman with whom she feels an instant connection. While the New York City commuter train is stalled, Selena and the woman, who says her name is Martha, begin to talk over mini bottles of vodka. Martha confesses that she is having an affair with her boss.

The need to talk, to tell somebody her secret, has Selena saying that she suspects her husband, Graham, is sleeping with their nanny, Geneva Markson. In truth, Selena knows about the affair as she was just watching them on the hidden camera installed in their sons’ playroom.

Firing the nanny should be the logical action, but, frankly, Selena values Geneva much more than she does her husband who, since he lost his job, has been unbearable. Geneva is well-organized, caring and a loving nanny to the Murphy boys, ages 5 and 7. A few days later, Geneva disappears. Selena then begins receiving text messages signed Martha “from the train.”

The train trope has been the background for many mysteries, Lisa Unger makes this setting all her own in her innovative “Confessions on the 7:45.”