Joey Harker teams up with all of his alternate selves from parallel universes to fight a battle between science and religion. Joey Harker, a teenage American boy, finds out that he is not alone in the altiverse, a word used in the novel to describe all the universes that make up the cosmos. There are hundreds and hundreds of versions of Joey, just like him, and they all live in different universes parallel to his own (and each with a variation on his name: Joey, Jay, Jake, et cetera). Joey finds out the strange truth of his (and their) existence, after participating in a town-wide science experiment led by his teacher Mr. Dimas in his hometown of Greenville.
Joey gets lost in a dense fog. He finds his way back home, but he discovers that his bedroom is occupied by a girl doppelgänger of himself (named Josephine), and no one knows who he is. He is afraid and runs into the fog again, but this time he goes to his school. Mr. Dimas is horrified to see him, claiming that he drowned a year ago. Mr. Dimas says he drug him out of the river himself. Perplexed by this confrontation with his own death, Joey is distracted by a character named Lady Indigo and a bizarre fiend named Scarabus (with tattoos that appear to be alive). The evil duo kidnap Joey and put him aboard their starship called the Lacrimae Mundi (which in Latin means "Tears of the World").
On board the starship, Joey learns that he is being taken to a place called HEX Prime. The starship travels through a region that is called the Nowhere-at-all. Fortunately Jay, one of Joey's doppelgängers, arrives on the scene, and through a bit of sci-fi magic, escape into a multidimensional space called the in-between. Joey learns from Jay that there are others like himself, and they have a special power that enables them to walk between worlds (that's why they're called Walkers). The in-between is the world in-between the worlds, and it is mainly populated by creatures that look like they belong in an oversized Atari game. These video game like geometrical creatures that inhabit the in-between are called mudluffs. Joey learns that he is part of a group of people who are trying to keep the balance between the dimensions. They all belong to an organization called InterWorld. There is the scientific side (the side that Joey and his look-alikes belong) and the Magic side, or what I call the religious side. Either side cannot overtake the other without disastrous consequences. Joey finds himself in the classic fictional battle of good versus evil.
Jay is attacked by a multidimensional snake creature and dies. He tells Joey the way to get to their headquarters (called Base Town) is via a mathematical formula. Joey befriends one of the mudluffs, one of the good multidimensional creatures, and names him Hue. He uses the formula to arrive at the headquarters and learns the advanced training of a Walker, an individual who can go in-between worlds. After receiving advanced training, Joey gains the respect of the other Joeys, even though they at first thought he was responsible for their buddy Jay's death.
Sent on a mission to a look-alike earth that turns out to be a trap set by Lady Indigo and her minions, all of Joey's friends are captured. Joey, defeated, but unharmed returns to base camp. The director, an older version of himself named Joe, wipes Joey's memory and returns him to his previous life. Joey completely forgets everything that ever happened to him in his life as a Walker until one day after blowing bubbles with his brother, he remembers the mudluff Hue and it all comes back to him like an involuntary memory. Galvanized with this knowledge, he returns to the alitverse to save his teammate doppelgangers who are still entrapped by the evil Lady Indigo and her magical gang of hooligans.
Since Joey's memory was erased, he has to tap into the deepest recesses of his recollections to reconstruct the way back to the ship where his friends are kept prisoner. With memories of Hue and the former Jay in his head, Joey finds the ship, and is captured by its commander, an ugly goblin looking creature named Lord Dogknife. Jay learns the fate of Walkers. They are boiled down to use their souls as fuel to run the ship.
Jay knocks over the pot of soul crushing poison and kills a couple of guards. He finds his friends alive and also releases the souls of other Walkers who have been trapped in bottles to fuel the ship. The ship's energy supplies run low, and Joey has to save the mudluff Hue. A version of Joey that appears to be an artificial intelligence, kills Scarabus, and in the final battle the released souls render Lord Dogknife inadequate to fight. But, Lady Indigo tries to use her magic potion to disable Joey and his gang. Remembering the equation to go home, Joey and his people escape and return back to InterWorld headquarters. The old Joe remonstrates the team for their waywardness and prepares them for the next mission.
Best part of story, including ending:
I liked the concept of multiple versions of the main character, but the overall story was silly, and I thought there was too much scientific explanation and not enough character development. The villains seemed cartoonish, but I liked the relationship between Joey and the mudluff creature named Hue.
Best scene in story:
I liked the scenes with Joe and the older version of himself. The old man Joe is tough and is slow to show appreciation. I was surprised when he erased Joey's memory, but I realize that in the end the older Joe was a good man. The older Joe is an older and wiser version of all the younger Joe's, Jake's and Josephine's that I thought he deserved his own story.
Opinion about the main character:
Joey Harker is a tool to drive the plot. The novel is more about the idea of an altiverse, and is not so much concerned with character development. I disliked how we never really got to know the personality and psychology of Joey Harker.