Rachel Weisz stars as a dangerously appealing widow in a spotty adaptation of Daphne du Maurier’s “My Cousin Rachel.” The unreliable narrator, Philip, tells us in his reliably drowsy narration that he will never know the truth about his cousin, and neither will we. The “my” of the title is very important, because it is through Philip’s inexperienced and selfish eyes we view the events of this watchable, but inessential, period film.

Sam Claflin (“The Hunger Games”) plays Philip, an orphan raised by his beloved older cousin, Ambrose. Just when Philip is about to turn 25, come of age and ascend to his inheritance of Ambrose’s estate, the older man falls ill and travels to Italy for a cure in the sunshine. Soon he is writing giddily of his vast improvement in the loving arms of their distant cousin, Rachel, who he impulsively weds.

Back home, Philip and the family lawyer (Iain Glen from “Game of Thrones”) are shocked by the reports they are getting from Ambrose, and suspicions mount when he suddenly feels much worse and suggests Rachel is a monster who watches his every move. Finally he pleads for Philip to come to Italy and rescue him, but Philip arrives too late and finds his beloved cousin dead and buried.

A sinister Italian guy explains that Ambrose died of a brain tumor, but Philip is unconvinced, to say the least. He also blames the mysterious Rachel and vows, aloud, to avenge Ambrose’s death via revenge against Rachel. The Italian guy seems unimpressed, and this is a completely accurate appraisal.

For soon Rachel writes and announces her intention to visit the estate and, while Philip pouts that this will be his chance to take matters into his manly hands, he is completely undone by Rachel within a matter of minutes. The degree to which he changes his tune is honestly preposterous, and strains credulity. Plot twists are one thing, but some element of reason should be maintained or the story becomes meaningless.

Weisz’s portrayal is wonderfully nuanced and intelligent, and her ambiguous character and motives (and fabulous costumes) are enough to float this gothic mystery even when the plot holes are gaping. I don’t know if Claflin was doing a great job playing a petulant idiot, or if he himself is just a dopey actor. I do think a bit more range on his part would have enhanced the story. We’re not sure how we feel about Rachel, but there’s no question Philip is a fool.

While we are left scratching our heads as to whether Rachel is trying to kill Philip, it’s more disappointing that she doesn’t succeed. Weisz makes us care about her past and current actions, but Claflin keeps us from caring too much about the result. It seems so obvious that she is, for example, poisoning him, but Weisz still manages to string us all along until we really don’t know. All we know is he kind of deserves it, just for pure naiveté and nonstop sulkiness.

Suffice to say, this doesn’t approach an adaptation like Hitchcock’s “Rebecca” in terms of a du Maurier adaptation, although it does have some of the signature techniques, such as the doubling of characters, which, again, could have been explored more skillfully. It’s also not as good as “My Cousin Vinny,” but what is?

The title character makes this a decent Gothic period film, but a male character who even approached her could have made this film so much better.

“My Cousin Rachel” is currently available to rent.