The novel is about two twins, Barry and Harry, who semi-inheritnote some property after their eccentric great-uncle dies. They discover that a metal shack (called the Playhouse) outside their uncle's home has a strange Year Inside, Hour Outside effect because something about that area acts as a portal to another universe.
This novel is an example of the following tropes:
- Always Identical Twins: Barry and Harry, of course.
- Dark Is Not Evil: The scary-looking toothy thing is just coming through the Portal Pool to destroy the portal to put an end to its dangerous effects, then eat itself alive before it can cause any harm itself.
- Diary: Harry keeps one while spending a year inside the Playhouse.
- I Just Want to Be Special: Harry. He doesn't want supernatural powers per se, but he wants to be different from his brother and to step out of his shadow. He does.
- Improvised Training
- More Teeth than the Osmond Family: The creature coming through a Portal Pool from another universe. It was visible from this universe long before it arrived, and the only part of it that was visible was its huge mouthful of teeth. The main character, in fact, spent a whole year waiting for the creature to arrive, knowing only its toothy appearance.
- One-Word Title: The book is presumably named for the black hole-ish nature of the Playhouse, and a singularity is the supermassive center of a black hole.
- Out of Time, Out of Mind: Averted. The protagonist spends about a year inside a timewarp, which is about 8 hours outside. Changing his personality was his goal going in, and it works, especially because the timewarp is only about 8 feet across, and he's the only person in it.
- Plot-Relevant Age-Up: Some cows wandered into the spot in the pasture where the Playhouse would be built and matured to old age overnight. Harry and Barry's uncle was also implied to have done this one night. Also Harry does this.
- Polar Opposite Twins: Harry and Barry.
- Portal Pool: When filled with water, the sink in the Playhouse shows images of another universe.
- Red Oni, Blue Oni: Athletic, popular and pushy, Barry, is Red Oni to timid, book-smart and rational, Harry's Blue Oni.
- Two Guys and a Girl: Harry, Barry and Lucy.
- Year Inside, Hour Outside: Time on the "outside" passes extremely slowly while in the Playhouse.