It is easy to want to rally behind "XX," but the film's novel hook—it's a horror anthology written and directed entirely be females—isn't quite enough to smooth over a problematic structure and the unevenness of its segments. Of the four stories, the strongest two burst out of the gate up front before the less inspired second half peters to a diminishing close. Were the superior tales bookending said weaker sections, perhaps it wouldn't have called as much attention to itself. As is, only the wickedly good opening 40 minutes of "XX" are worth the viewers' while.

Jovanka Vuckovic's "The Box," based on a story by Jack Ketchum, is exceedingly unnerving, made all the more haunting by its refusal to provide easy explanation to the harrowing turn of events. Danny (Peter DaCunha) is riding home on the metro after a day spent in Manhattan with mother Susan (Natalie Brown) and older sister Jenny (Peyton Kennedy) when his curiosity over the contents of a passenger's (Michael Dyson) red gift-wrapped Christmas box gets the best of him. Immediately upon seeing what's inside, he loses all interest in eating. As the days pass and an increasingly emaciated Danny refuses every meal, dad Robert (Jonathan Watton) grows increasingly concerned and Susan goes into denial that her son has a problem. Like a fast-moving train with faulty brakes and a cliff up ahead, "The Box" is mesmerizing in its sustained ambiance of dread. In twenty minutes, Vuckovic tells a fully formed story of a family's rapid dissolution, light on exposition and heavy on fearful, foreboding suggestion.

Making her directorial debut, Annie Clark (better known as musician St. Vincent) is up next with "The Birthday Party." Melanie Lynskey (2016's "The Intervention") is deliciously amusing as Mary (Melanie Lynskey), an overachieving mother determined to give daughter Lucy (Sanai Victoria) the birthday celebration of her dreams—even after she discovers her husband slumped over dead in his office. Raven-toned humor and macabre tension lead the way in "The Birthday Party," culminating in a bleak zinger of a final scene as wrong as it is so very right.

Roxanne Benjamin (2016's "Southbound"), who co-wrote "The Birthday Party," has less luck with her own segment, "Don't Fall." Feeling noticeably more rushed and thrown together than the more meticulously woven previous two cinematic morsels, this story of four college friends (Angela Trimbur, Breeda Wool, Morgan Krantz, Casey Adams) vacationing at a desert campsite who are stalked by an ancient evil is anticlimactic and awfully familiar. When it's over, the immediate reaction is, "Is that it?"

Finally, as flat-out brilliant as her 2016 thriller "The Invitation" was, director Karyn Kusama's "Her Only Living Son" simply doesn't have the same ominous punch. As son Andy's (Kyle Allen) 18th birthday approaches, Cora (Christina Kirk) grows alarmed by his increasingly violent tendencies. Concerned that he may be exhibiting signs of mental illness, she moves closer to the truth involving Andy's nefarious conception and devilish destiny. With shades of 2011's disturbing "We Need to Talk About Kevin," "Her Only Living Son" is promising in concept until it reveals a disappointingly commonplace hand of cards. The payoff is especially unsatisfying.

The danger with any filmic omnibus is the inconsistency between one story and the next. For an anthology to work best, the filmmakers must make sure quality is roughly even across the board, with equal care brought to each interlude. Had "XX" kept up the pace of its first two exquisitely executed yarns, this might have been the best anthology since at least 2012's "V/H/S." As is, "XX" is only half of a good movie. Like 2016's equally spotty but occasionally chilling "Holidays," the lesser parts serve to undermine the great ones.