It’s always risky to mix genres, but some movies pull off the trick with more skill than others. Dragonfly, which had its world premiere this week in Tribeca, starts as an intimate character drama, with two Oscar-nominated British actresses — Brenda Blethyn and Andrea Riseborough — at the top of their game. (They were given a joint acting award by one of the juries at the festival.) Paul Andrew Williams’ movie unfolds at a fairly leisurely but rewarding pace until an event occurs that unhinges one of the characters and turns the movie closer to the horror genre, with a blood-splattered finale. Reactions will surely be mixed about this surprise tonal shift, but there is no doubt that the film sticks in the memory.

Blethyn plays Elsie, an elderly woman living in a drab housing complex and tended to by caregivers who do the bare minimum to meet her needs. Her next door neighbor Colleen (Riseborough) senses that Elsie may require extra help and tries to intercede, offering to do shopping and cleaning. Elsie’s son John (Jason Watkins) makes occasional appearances and seems suspicious of Colleen, but he clearly does not want to take on much of a caretaking role himself.

The friendship between the two neighbors makes us uncomfortable from the start, and the presence of Colleen’s large, menacing dog does not calm our fears. When Elsie gives Colleen money to do some extra shopping for her, we can sense that Colleen may have motives beyond pure altruism in looking after her neighbor.

The first part of the movie, sketching in the friendship, is paced a tad too slowly, but we are aware that the situation is unstable and the two actresses help to keep us riveted. Blethyn earns our sympathy without begging for it, and Riseborough is always commanding. Her surprise Oscar nomination for To Leslie a couple of years ago, aside from being the result of savvy campaigning, was confirmation of the skill that she has demonstrated over the last decade. (Her outstanding performance in a recent Masterpiece Theatre production, Alice & Jack, in which she costarred with Domhnall Gleeson, also was evidence of her vigor and versatility.)

The script for Dragonfly could have provided a bit more background for Colleen. We can understand that Williams didn’t want to construct a labored psychiatric case study, but he might have sketched in more of her history to prepare us for the shift that takes place in the final third of the movie. That includes a couple of gotcha shock moments that had the Tribeca audience literally gasping and screaming.

Is this tonal swerve a little gimmicky? Probably, and the film will not be to everyone’s taste. But it is a skillfully rendered exercise in terror. Williams has studied a couple of Hitchcock movies, and he has absorbed sly lessons from the master.

This is a slight film, but the jolts do stay with you, and the two stars offer a humanity that many horror movies lack. Some smart distributor should snap it up.