"Cleanup on aisle fifteen!"

"Ug, not the same dog again," I groan. The largest mastiff I've ever seen has left a lake on every aisle so far, and guess whose job it is to clean it up.

I'm only on aisle seven.

"Someday I'll invent a mop that does this by itself," I grumble, dunking my partner in its bucket and kicking it along to the next aisle. I haven't been in robotics clubs since middle school for nothing.

"But Gabby, then someone would be out of a job," Reese chides from atop a ladder as she rearranges clearance cat beds stacked to the ceiling. She throws me a mocking smile as I find the mess in aisle eight.

I ignore her and blow sand-colored hair out of my face as my mop splashes down on yet another yellow pond. "Why'd they even come down this aisle? It's only cat toys!"

The glass doors at the front of the store slide open, and I glance up, the mandatory, "Hi, welcome to Paw Paradise," on my lips, but the words freeze there, trapping my breath.

He stands in the entrance. Not just any he. He. Juan Martinez, the guy my ten-year-old mind dubbed the most beautiful person in the world when I first saw him. He lived next door. We walked to and from school together, stood alongside one another at graduation, and I haven't seen him since.

Now he shows up, five years later, wearing a Tron-inspired motorcycle suit, dark hair blowing in the breeze, honey brown eyes meeting mine, and I—

I have a mop full of dog pee.

I duck behind my bucket, which I am too big to successfully hide behind, my insides melting as he smiles crookedly and starts toward me.

You're a big girl, Gabriela. Stand up and face him like the strong woman you are.

I shoot to my feet, hands nervously unbunching my shirt under my apron's tie as he stops in front of me, smile wide as the sun.

His arms fly around my waist, lifting me off the ground, and I gasp. We had been friends, and I had written his last name after my first a million times, but this is a scene from my wildest dreams.

I completely miss what he says.

"What?"

Still holding me tight, he looks up, eyes full of shine and hope. "I finally found you, Grandma!"

I drop my mop.

"Did you just call me Grandma?"

He sets me down. "Gabriela Martini?"

"Yes, and I'm two months younger than you." My face scrunches. Is this some hipster fad, addressing people as a grandparent? A beat too late, I add, "Grandpa."

His brow furrows. "I'm not your grandfather."

"Well, I'm not your grandmother, Juan. That's impossible."

My gaze narrows. Up close, he doesn't match my memory exactly. He greatly resembles my Juan, but his jawline is different, more delicate.

"You are Juan Martinez, right?"

"Yep." He scratches the back of his head. "Juan Martinez III."

"You moved next door to me when we were ten?"

"I've never seen you before," he admits, head wagging. The sound of my heart exploding and raining down around us like gruesome confetti drowns out the rest. "This young anyway, not even in pictures. Grandpa was right. You were pretty."

"Were? Grandpa? What?"

Behind me, Reese giggles. "So, she's your grandma, not your girlfriend?" She twists a tight ringlet of ebony hair around her finger as she booty-bumps me aside. "Are you single? You're too hot to be single. Your feather earring is so cool."

As she bats her eyes, slinking in for the catch, I dive for Juan's hand and yank him down the aisle.

"Gabby, there's still pee on the floor!" she calls after me.

"It'll wait!" I slam open the door to the staff lounge/dog grooming room and swing Juan inside, making sure he stops well before the window where customers can watch their pooch get pampered in the tub in the far corner.

With my back to the closed door, I stand arms crossed as my sense finally settles into place. "This is a joke. Where's your camera? Are you Juan's brother? Cousin?"

Juan was adopted—the knowledge wriggles in the back of my brain—by distant relatives who looked nothing like him. He has no close family left. He never wants to talk about them.

The imposter shakes his head, lower lip caught in his teeth. "The Juan Martinez who grew up next to you is my grandfather."

"You're saying you're from the future," I deadpan.

"Not just the future." He pulls a click pen from his sleeve and draws in the air with luminous orange lines.

"Is that a hypercube?"

"I'm from the dimension we call the Inner." He points at one of the tesseract's vertical planes, then a perpendicular one. "This dimension is called the Outer. I'm here because I messed up everything, and to fix it, you have to rescue Grandpa."

I raise an eyebrow. "The Inner is ahead of the Outer in time?"

My mind whirs at Mach Two, pieces snapping in place. If my Juan is this guy's grandfather, and I'm his grandmother, then I must end up with Juan.

A million butterflies fill my stomach.

My grandson winces. "The dimensions aren't as neat as I drew them. They're more like this." He scribbles. "And they undulate, never meeting at the same point in time."

"So, it's like The Doctor says: Time really is wibbly wobbly stuff." My hands wring each other behind my back. I'm going to end up with Juan, and this guy is proof. I look for any of myself in him. Maybe in the pinch of his nose?

"What doctor said that?"

"Nevermind. How did you mess everything up?"

His eyes gloss over, falling to his polished boots. "We have a ritual. Nature demands balance between the dimensions. If someone steps into the Outer, someone there is pushed into the Inner and vice-versa. The Inner has far fewer inhabitants, and to bring diversity to our gene pool, we give up the brightest of each generation in hopes of getting someone just as desirable from the Outer."

He peeks up at me, and I recognize the expression. It's the one I see on my own face in the mirror when I'm about to break down, everything I wish I had never said running through my head.

"But you and Grandpa changed that. You invented gear, calculations, procedures that let someone cross over where and when they want. It's all regulated, with those who switch places knowing exactly where they're headed." He looks down again, a tear sparkling on his lashes, and my heart bursts.

I hug him. "You broke the rules?"

He nods. "I came to the Outer legally, but I met Gwen. She was wonderful. I loved her, and they refused to let me bring her back." His voice sinks deep, cold like a ravine at the bottom of the ocean. "I stole gear, but my calculations were hasty, and the moment Gwen stepped into the Inner, everything changed."

"Because her presence pushed someone important out of the Inner?"

He shimmies out of my embrace and meets my gaze sternly. "My grandfather. Because of that, Grandpa wasn't selected as a sacrifice. He wasn't adopted in the Outer by descendants of former sacrifices, and he never moved next door to you. He never met you. You never changed the world, and I was never born."