Zack had gone completely bonkers – too much homework will do terrible things to a boy’s mind. But then I grew suspicious. He knew how much I liked comic books and was constantly poking fun at me for what he called my childish obsession. I smelled an ambush.

“Superpowers?” I folded my arms and sneered. “What, so now you can fly and shoot lightning from your fingertips?”

A curious expression spread across his face. “I wonder,” he mused, sticking out one hand, flaring his fingers at me like some cheesy magician. Lightning did not shoot from his fingertips. But I was too stunned to notice since something equally remarkable was happening.

My torch flew out of my grip, spun through the air and landed in Zack’s outstretched palm with a slap. His fingers closed around it and he grinned.

Im-poss-ible!

But Zack had done it. He had made the torch move just by thinking about it and doing a lame hand gesture. Somehow it was true. My brother had an actual superpower!

Telekinesis, to give it its official title. Lots of superheroes have this ability in comics, but this was the first time I’d seen it in real life. I hated to admit it, but it was cool. Supercool. Not that I was going to tell Zack that.

“No lightning bolts then,” I said, pretending to be disappointed.

“What?!” He looked at me like I was stupid. “Did you see that? Did you see what I did?”

I couldn’t keep up the pretence – I was impressed. But my awe quickly gave way to something else. I was as green as the Hulk; more jealous even than last Christmas when Zack was given an iPhone, and I got shoes.

“It’s not fair! How come you get superpowers? You don’t even read comics.” I ranted for a few more minutes – when I get going I have been known to turn purple – and then, finally exhausted, I flopped down on the floor and felt my face crumple into a sulk. Although I was seething with envy I had to know. “How did it happen?”

Zack stared past me, his eyes fixed on some hazy spot on the wall, and began to describe the incredible – and incredibly recent – events.

“Just after you left I heard this distant rumbling noise and so I looked out of the tree house. There were lights in the sky and I thought it might be a meteor shower. And then I realised it was heading this way – fast. The sky was filled with hundreds of glowing white lines. But just as they were about to hit, they came to an almighty sudden stop. Then I saw that it was no meteor shower…”

He paused and drew a long breath before saying in a whisper, “It was a trans-dimensional spacecraft.”

I gasped. Up until then the most exciting story Zack had ever told me involved a bad haircut and a chihuahua. And I’m not convinced he was telling the truth about the chihuahua.

“It was a large blue oval hanging in thin air, right outside there.” He extended a trembling finger and pointed. “As I watched, a door in the side of the craft slid open with a sound like ‘bloop-whoosh’ and a luminous figure emerged on a beam of light. He wore a shiny purple suit, a cape with a high gold collar and gold boots. On his chest were three gold stars that pulsed like heartbeats. He had a dome-shaped head, which was completely bald, and a wispy beard that he stroked when he spoke. He gave me a three-fingered salute and introduced himself as Zorbon the Decider, an inter-dimensional traveller and representative of the High Council of Frodax Wonthreen Rrr’n’fargh. Everything he said sounded like he was talking in capitals. Zorbon explained that he came from another universe that exists in parallel to ours. It’s almost exactly the same as our universe, he said, except there the colours green and red are reversed, and sponge cake tastes different.” Zack looked thoughtful. “Not entirely different, just a bit different.”

I could tell by his daydreamy look that Zack found this boring fact particularly fascinating and there was a significant danger that he’d carry on talking about sponge cake. “Never mind about the stupid cake! Get to the superpowers!”

Zack shook himself out of his trance. “Oh yeah. Well, Zorbon said that I’d been chosen by the High Council for a mission of utmost importance to both our universes. A mission so vital that were I to fail, the consequences would be cataclysmic for trillions of beings.”

“Two universes? You have to save two universes?” Typical. My brother couldn’t be happy saving just one. He was such an overachiever. “But why you?” I wailed.

Zack stared thoughtfully out of the door. “Apparently this tree house is the junction between the two universes.”

This was incredible. Mind-blowing. Our tree house, a portal between two worlds. On the other hand… “So?”

Zack shrugged. “I guess I was the first person Zorbon met when he came through.”

I was speechless. My mouth moved but no words came out, just a sound like air escaping from a balloon. That’s not how you choose a saviour of mankind. There has to be at least a prophecy written in an ancient book. This was like giving the Sword of Ultimate Power to a goldfish.

“To ensure my success,” Zack continued, “Zorbon said he was authorised to bestow upon me six gifts – powers, if you like – to aid me in my cause. Then he raised his palms, said something in this really weird alien language –”

What, as opposed to a really normal alien language? I thought but didn’t say anything.

“–There was this flash of red light – or maybe that should be green light,” Zack went on. “I felt a surge of energy through my whole body. Every atom of my being was on fire. When it finally stopped, Zorbon bowed and said, ‘IT IS DONE.’ I asked him what was done? What powers had he bestowed? What was my mission? ‘I MUST NOT SAY, FOR IF I DO I RISK ALTERING THAT WHICH IS TO BE. AND AS ANYONE WHO UNDERSTANDS THESE KINDS OF SITUATIONS WILL TELL YOU, THAT WOULD BE A VERY BAD THING. ALL WILL BECOME CLEAR. IN TIME,’ he said. Then he gave me this enigmatic smile and left. But just before the door of his craft slid shut he said there was one thing he could tell me. This really scary look came over him and he said, ‘NEMESIS IS COMING.’ And then he was gone. Bloop-whoosh!”

I stood there with my mouth wide open. So much to make sense of. So many questions. However, one thought pushed its way to the front of the queue. “But I was only gone five minutes!” The most important five minutes in the history of the world and I’d missed them because I needed a wee. “I bet if I’d been here, Zardoz the Decoder would have chosen me,” I grumped.

“His name was Zorbon the Decider. And you weren’t here.” Zack shrugged. “Should have held it in, shouldn’t you?”

It was so unfair! I was way beyond acting like a normal, sensible person. “Get him back. Tell Bourbon the Diskdriver he made a mistake and he has to come back and give me superpowers too.”

“Zorbon the Decider,” corrected Zack once more. “And he decided I was the one. Not you.”

“I don’t believe you. We can’t know for sure unless you call him.”

“Call him? Oh yeah, because he left his phone number. Uh, what’s the code for the parallel universe again?”

I detected a note of sarcasm in the question. Zack was teasing me, which was a rash thing to do given that at that very moment I was more furious than I’d ever been in my entire life.

“What are you doing now?” he asked.

I stalked round the tree house, tapping the walls every few feet. Wasn’t it obvious? “Searching for the portal to the other universe.” I pressed one ear to the back wall. “I think I can hear it.”

“Luke.”

“Shh!” I hissed. I could definitely make out a sound. “Yes. Something’s coming through. Sounds like scratching. Could be inter-dimensional mice.”

“Uh, Luke…”

I spun round. The scratching sound was coming from Zack. He clawed at his chest through his shirt. As usual, he was wearing his school shirt because, he said, it put him in the right frame of mind for homework. (I know. And I have to live with him.) Something weird was going on underneath. I screwed up my face and pointed. “What’s that?”

A soft glow pulsed beneath the material like a night-light. He popped the buttons, gripped each half of his shirt and pulled it apart to reveal his bare chest beneath. I swear I could hear trumpets.

Despite what Dad had said, there was no hair, but there was something else. Inked across his chest were three glowing stars.

“Zorbon had stars just like these,” said Zack. “I wonder what they mean.” He ran a finger over them.

“I’ll tell you exactly what they mean. They mean you’ve got a tattoo.” I shook my head. “Mum’s going to kill you.”

Zack ignored me. He straightened, drawing himself up to his full five feet and three inches, and a calm, thoughtful expression came over him. “I know what the stars mean,” he breathed. “I. Am. Starman!”

I raised a finger of objection.

“What?” he snapped.

“Uh, sorry, but there’s already a Starman. You’ll probably get sued.”

Zack gave a huff of irritation. “Fine. Whatever.” He drew himself up again. “I. Am. Star Boy!”

He swivelled his eyes towards me, just to make sure. I gave a little shake of my head.

He threw up his hands in frustration. “There’s a Star Boy too?”

“I’ve told you a million times, you should read more comics.” I tapped my cheek thoughtfully....