“A Bigger Splash,” by director Luca Guadagnino, floats along for 90 minutes as a relaxed exploration of four quirky characters — attractive, lustful, bored, somewhat confused — and the shifting ties that bind them, carnally and otherwise. Then, suddenly and shockingly, it turns into a psychological thriller for the last half-hour. It’s a shift in tone that feels uneasy and a little forced.

The setting is the Sicilian island of Pantelleria near Tunisia, where Marianne (Tilda Swinton), a rock star recovering from vocal surgery, and her boyfriend of six years, Paul (Matthias Schoenaerts) are staying in a sprawling home atop rocky cliffs, with a nice swimming pool. This means that for most of the film, Swinton lacks one of an actor’s chief instruments: the voice. This challenge suits Swinton to a T; in fact, it was her idea to render Marianne virtually speechless, jettisoning dialogue originally planned for her.

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Their idyllic vacation is disrupted when Harry (Ralph Fiennes), a gregarious record producer and Marianne’s former flame, arrives unexpectedly. As if his uninvited visit isn’t enough, Harry’s brought along a surprise: a 20-something daughter, Penelope (Dakota Johnson), whom he’s only recently met. Fiennes is uninhibited, both physically and emotionally, and quite riveting as Harry, a man burning with unresolved appetite.

Watch him sing sexy karaoke with Penelope — yep, sexy karaoke with his daughter. Watch him try to woo Marianne back with the help of some warm, freshly made ricotta cheese. And finally, watch Fiennes dance rapturously to the Rolling Stones’ “Emotional Rescue.” It’s a cliché, but we’ll say it: It’s worth the price of admission.

After 90 minutes on simmer, our four-burner stove goes straight to boil. A relationship sours, to stunning effect. Despite the feel that we’ve entered an entirely different film, it’s hard to deny this is the most exciting part.

Well, almost. Have we mentioned Fiennes dancing to “Emotional Rescue”?