ALA Booklist (Sun Sep 01 00:00:00 CDT 2013)

When a book's first paragraph includes the terms photo laser module, beam splitter, and vacuum dome, you know you are in for some geeky sci-fi fun. Josie, 16, is the geek in question, a mostly ignored high-school brain whose mother is known for her work on creating micro black holes. Then an experiment goes ess what? rribly awry, and Josie begins seeing her twin, Jo, from a parallel universe via an antique mirror. Twice a day, at exactly 3:59, the two can communicate. They decide to step through and swap lives for a day, a fine idea until Josie discovers that Jo's world is infested with the Nox, vicious monsters that feed on humans at night. Maybe even worse, Jo is in no hurry to come back home. Logic can get a bit strained, and the science is kinda goofball, but McNeil's movie-quick setup speed and humorously inventive language (Josie needed to spend some time perving around the house) make her a very welcoming author who succeeds at injecting fresh verve into some stalwart sci-fi concepts.

Horn Book (Tue Apr 01 00:00:00 CDT 2014)

Josie discovers a parallel world accessible every twelve hours at 3:59, and, running from a painful breakup, switches places with her counterpart, Jo. Jo's world, however, is overrun by deadly night-monsters possibly connected to Jo/Josie's mother's quantum experiments. Josie's romantic concerns are soon tiresome in the face of life-or-death peril, but the bait-and-switch plot, especially the parental doppelgangers, offers ample thrills.

Kirkus Reviews

In this quasi–science-fiction novel, doppelgangers Josie and Jo agree to temporarily trade places after their mothers' scientific experiments accidentally open a portal between their parallel worlds. The situation soon spirals out of control as Jo's mother (who also secretly traded worlds) attempts to make the trade permanent by destroying the portal. McNeil deserves credit for writing science fiction featuring women scientists. Josie and her lab partner, Penelope, as well as Penelope's parallel-world other, Pen, all casually drop quantum-mechanics and parallel-universe theories and use the scientific method to understand the portal between their worlds. However, the devil is in the details, and offering little explanation of the scientific theories under discussion compromises the novel's scientific tenor. Further stretching the science is the improbable existence of the laser that is used to create the portal. Readers must accept that an X-FEL laser, "one of the most high-tech, cutting-edge pieces of equipment in the world," whose production at a lab required "millions in funding, a team of A-list scientists and engineers, top secret specs no one had ever seen," was secretly recreated out of scraps in a residential basement. Also implausible is the half-baked insta-romance (true love after four days, really?) that fails to create romantic tension. The patchy science, though hastily injected with romance, makes for an unsatisfying read. (Science fiction. 12-18)

School Library Journal (Fri Nov 01 00:00:00 CDT 2013)

Gr 9 Up-Josie Byrne, a brainy prep school junior, has been brought up to trust in science. Her AP physics project examines the Penrose Interpretation, which hypothesizes that objects can exist in more than one space at the same time. Josie's mind isn't always on science, however: her job at Coffee Crush, her parents' separation, and her painful breakup with Nick, not to mention mysterious deaths in the neighborhood, occupy her thoughts. When an unusual set of events line up precisely at 3:59 a.m., Josie finds a door that opens to a parallel universe, but only for a moment and only twice a day. In this other world, steadfast Josie exists as Jo, a spoiled young woman who still appears to have Nick's love. But Jo's existence is far from fun and games, despite expensive clothes and a BMW. While the same people live in both worlds, their personalities are different. Even worse, Jo's world is inhabited by Nox, creatures that make it deadly to be out at night. The story focuses primarily on Josie, Nick, and the Nox in one dimension, along with people who have transferred between the two universes. Parental love, double-crossing, and the nature of friendship all have a place in the plot. To fully move into Josie's world, readers need to embrace quasi-scientific jargon and believe that teens could construct and operate a version of a multimillion-dollar, top-secret government laser, in a basement, using mirrors held by hand. Those looking for romance/adventures set in parallel universes will come away satisfied. Maggie Knapp, Trinity Valley School, Fort Worth, TX