There is no way to completely discuss “Twinless,” the latest film from writer/director/actor James Sweeney, without robbing its biggest turns of their impact. The initial premise, about two guys that meet in a twin bereavement support group and then start to grow closer, is merely a small fraction of what this film has in store as it upends expectations and runs with them as far as it can. It’s a juggling act of tones that manages to be funny, chaotic, dark and even unexpectedly poignant. The film also has Dylan O’Brien giving one of his best performances to date, bringing just the right amount of heartfelt himbo energy to his role as grieving twin Rocky and giving the film unexpected emotional weight in key moments.

The film, which premiered Thursday at Sundance, begins with a car accident occurring offscreen. Putting its darkly comedic tone on immediate display,...