Max's best friend Zack shows up armed with a lousy present (some used X-Force comics). Also at the party are two girls. Erin, whose voice sounds like a mouse, and April. Max excitedly tells his guests that they're gonna "barbecue"-up some hot dogs. Max tries to get everyone to watch "the Terminator movie" he rented.
After the party dies down, only April and Erin are left. The four kids decide to explore a dusty old attic. Max's dog Whitey discovers a hidden room in the attic. The room is empty except for a large antique mirror that takes up almost the entire wall. A slender light rests on top of the mirror and when Max pulls the light's chain, the light comes on but he disappears.
After getting one of the other kids to pull the chain and turn the light off, Max reappears. Max doesn't believe he really got invisible. Erin and April's ride arrives and Max tries to put the whole ordeal behind him. Yet that night he has trouble sleeping and sneaks up to the attic to investigate. Max slowly takes an inventory of the mirror's construction when he spies an ominous figure behind him. He turns to discover it’s only Lefty.
Lefty whines until Max agrees that he can go invisible only if they both go invisible at the same time. They both go invisible for a couple minutes, start to feel weak, then pull the switch again. This time however it takes longer to reappear. Max figures that it must have something to do with how they were invisible longer than Max was the first time.
The two brothers decide the best way to use this amazing new power of invisibility is to scare Zack, who wasn't there for the prior mirrorfest. And eventually they get him over to the house and they do scare him. Then they invisible him. Then April and Erin show up. Erin also wants to get invisible, but April remains hesitant and instead times everyone's excursions into invisibility. The kids will all attempt to stay invisible the longest. Before Lefty can take his turn though, his grandparents arrive for dinner and the other kids have to leave. But they agree to meet again later in the week to settle the contest.
While at dinner Max sees a spoon raise in the air. Max wrestles with the invisible Lefty and drags him upstairs to the attic. Lefty brags as he comes back into sight that he stayed invisible for over ten minutes, so now he holds the record. Lefty promises not to use the mirror again.
That night, Max stays up late because of the weekend and hopes that his parents will let him stay up and watch Saturday Night Live. Erin calls and tries to convince Max that if they brought the mirror to school for the science fair, they'd be sure to get first prize and appear in People. Max tells her that it's a bad idea because the wrong people could find out about the mirror, like the military.
Max sneaks off to the attic. He sits against the wall opposite the mirror and stares at his reflection. As he starts to doze off, he's startled to hear a voice in the mirror, asking him to change his ways. Max runs down to his bedroom and hides underneath the sheets until morning.
When Max awakens, he soon discovers that Lefty is invisible again and this time using invisibility to float a shirt in Max's room. Max makes his brother go upstairs and change. When he comes back down, Max and his mother notice he's acting a little strange and there's something not quite right about him.
Max tells Lefty that he's decided getting invisible is too dangerous and calls his friends to cancel the invisibility championship. However, when the day of the championship arrives, Zack and the two girls show up at Max's house anyways. Turns out Lefty, who is currently hanging out at the park with his friends, called them up and uncanceled the championship. Max decides that as long as they're all there, they might as well go up to the attic.
April finally gets to disappear but since Max's dog accidentally gets invisible with her, he brings her back early. She's upset but is again relegated to taking the time for the others. Erin goes next and for the first twelve minutes or so she's up to her regular hovering objects shenanigans. However, for several minutes they can't get her to respond to their questions. Eventually they spy a floating Coke can and Max brings her back by turning on the light. After she comes back into focus, she claims she just got thirsty and went downstairs and that's why she didn't reply. However, Max notices something odd about her appearance too.
Zack decides he's going to smother the record and doesn't want to be pulled back into reality until after fifteen minutes have passed. He then takes off outside and pulls a hilarious prank on an older neighbor. He picks up tomatoes and he makes them hover in the air. Once his friends stop laughing at his riotous object floating, they notice that he too does not reply to their calls. Against Erin's objections, Max races upstairs and turns the light off.
Zack reappears and Max can't help but notice that his haircut has now been reversed. Max tries to say something but Erin and Zack push him under the light of the mirror and turn it on. He decides to not go anywhere and simply wait out his turn. As the time presses on, he begins to feel weaker and a white haze begins to envelop him. No matter how hard he cries, they won't pull the switch.
Now fully surrounded by white light, he sees an object floating towards him: his living reflection! The Reflection Max tells Actual Max that the reflections have already taken over Erin and Zack and now it's Max's turn. Max doesn't think living for eternity in a mirror sounds so hot, so he escapes. Somehow he manages to outrun his reflection and appears in the flesh once Erin and Zack pull the cord. They ask him if he's made the switch and he says nothing. Then the Reflection Max gets seen in the mirror and tells them that Actual Max got away.
Erin tells Reflection Max no problem and Zack pulls the cord again. Actual Max tries running out of the attic but Erin and Zack block his way. He yells to April to go get help but she gets blocked by Erin and Zack. Just when all hope is lost, Lefty appears in the attic, sees the reflection of the invisible Max in the mirror and tosses him the ball. The ball shatters the mirror, sucking Max's reflection into the shards. Lefty holds onto the door frame while Erin and Zack also get sucked into the mirror and then spit back out again. Max and Lefty reflect on the end of their adventure while tossing around a ball in the backyard. It’s then that Max realizes Lefty is throwing right-handed.