Police detective Trav Becker, on suspension during an investigation into the death of his young partner Adam, wakes up after the latest in a series of benders, which has caused nearly every one of his friends, including his long-suffering girlfriend, Mary, to desert him. Only Sam Markus, physicist and Trav’s best friend since college, is still coming around. On this particular morning, the day that Trav will learn the verdict of the Board of Inquiry, Sam urges Trav to go into work and learn the results in person.
To Trav’s shock, he not only has been cleared of all charges, but gets a commendation for bravery. And to top it all off, Adam is alive.
But after the initial shock wears off, Trav strangely doesn’t question the change in his fortunes, even treating his partner to breakfast at their favorite spot. Adam, as usual, is tongue-tied when he encounters Kim, a waitress on whom he has a serious crush.
Trav visits Sam’s laboratory, where his friend explains his current line of research. Sam is very close to proving the existence of parallel realities. On a lark, he scans Trav and discovers that he originated in another universe!
According to Sam’s theory, Schröedinger’s famous thought experiment involving a cat in a box filled with poisonous gas, whose quantum state is both living and dead until the box is opened, plays out for real for all of us every day. Something as simple as finding your car keys in a place you could have sworn you already looked is an actual, real-world example of how we slip between adjacent, parallel realities. Our brains smooth over any inconsistencies brought about by these shifts.
Sam assures Trav that even though it may seem a little weird, what has happened to him is not that out of the ordinary. There is no returning to the stream where Trav originated, “No backsies,” as Sam puts it. He encourages his friend to just behave normally.
Trav returns to work and is called to a stabbing at a seedy bar called The Kremlin. It’s in a former industrial area that is just beginning to see some development and gentrification, but not so much that the original, rough clientele has moved on.
One of the witnesses Trav questions, a woman named Morgan Foster, who gives her occupation as psychic, turns the questioning around. She is very curious about Trav, as if she can tell there is more to him than meets the eye.
But before he can talk further with her, a snippet of memory from his previous reality (or “stream,” as Sam calls it), reveals the probable identity of the killer to him, an Uzbek thug named Bilol Grymzin.
At the police station, Trav encounters Grymzin’s boss, Anton Kaaro. Also an Eastern European immigrant, Kaaro runs most of the organized crime in Trav’s community, under the guise of legitimate businesses like The Kremlin.
Accompanying Kaaro’s visit is the news that a confidential informant within Kaaro’s organization, a bartender at The Kremlin named Rob Lennox, has gone missing. This may have been Trav’s fault, as he accidentally mentioned an informant when he was at the bar. Lennox’s handler, an officer named Amy Harper, has lost contact with him and is concerned. Trav is given the assignment of helping Amy figure out what is going on.
As he prepares to begin the search for Lennox, Trav is perplexed when he encounters a room full of ghostly versions of himself, all appearing to go about their normal business. He realizes that he is somehow seeing across the different reality streams Sam described. While pondering this, Trav gets another surprise, this one very welcome. Mary shows up at the police station with a copy of a record album as a gift for him. When she comes eagerly into his arms, Trav realizes that on this stream, he and Mary are very much still together. Trav’s new “home” is starting to feel like a pretty good place.
That is, until Sam shows up, wearing different clothes and having acquired several days’ growth of beard. He demands that Trav accompany him to Trav’s apartment. Once they arrive, Sam reveals that he, too, is from a parallel universe. In fact, it was this Sam who drove Trav to work that morning, in the process orchestrating their shift to a stream where Trav’s situation is so much improved. Sam explains that he comes from a stream where his work on parallel universes was much more advanced. Trav mentally designates these versions of himself and his friend, “Sam Zero” and “Trav Zero.”
But something went wrong. The knowledge that Trav could influence events caused him to come unhinged. Unable to stop Sam’s work, Trav took it upon himself to rid the Multiverse of as many versions of himself as he could, lest they upset the balance of reality. As proof, Sam opens Trav’s closet, revealing the body of this stream’s original Trav Becker, dead from a gunshot wound.
Sam Zero goes on to say that he has been following a trail of Dead Travs for days, trying to get “ahead” of Trav Zero and prevent him from killing again. But before he can help Sam, Trav is kidnapped at gunpoint by Bilol Grymzin, now out on bail, and taken to meet with Anton Kaaro.
Trav is shaken to discover that the original, now deceased Trav from this stream (which he now designates “Stream One”), was on Kaaro’s payroll. Grymzin, who does not trust Trav, threatens him, but is unable to follow through because they are interrupted by the sudden appearance of Morgan Foster. Cleverly helping Trav extricate himself from Grimzin’s clutches, Morgan tells Trav that she knows he is from another “plane” of reality, and calls him a Traveler.
Morgan reveals she once researched unexplained phenomena, and that there are many examples throughout history of individuals suddenly claiming to have radically different memories of people and events, as if they had experienced an alternate history. She believes that many people through the ages have exhibited similar powers to Trav’s, which is the origin of the legends of witches and wizards.
Trav connects with Sam Zero, who offers to help him travel to Stream Zero with a special app for his smartphone that will enable Sam to retrieve him. But something goes awry, and instead Trav ends up on the original stream where this all started, which Trav now calls Stream Four, and where Trav was a broken, suicidal, ex-cop. Trav overhears that the body of Rob Lennox has been found at an abandoned warehouse owned by a company associated with Anton Kaaro.
After being kicked out of the station, Trav encounters someone trying to break into his car. Too late, he realizes it’s Trav Zero, who knocks him unconscious.
Trav awakens tied to a chair in his own apartment. Trav Zero appears and says he knows that Sam Zero has recruited Trav to stop him. But he also seems to know that Trav doesn’t know enough about his Traveler powers to use them. Zero intends to strand Trav here, in his home reality. But fueled by his frustration at being manipulated by the Zero versions of both Sam and himself, Trav unlocks just enough Traveler mojo to get free. He seeks help from the only person he can trust, the Sam Markus of his home stream.
But reverse engineering the work of a Sam from another stream is simply going to take time that Trav doesn’t have. With the scientific path to shifting streams denied him, the only option left for Trav is the mystical one. And there is only one other person who seems to understand that aspect: Morgan Foster.
For the second time in a few hours, Trav has to convince someone that his story is not crazy. But Trav knows things about Morgan he couldn’t know unless they had met previously. And Morgan’s cat treats him like a long-lost friend. Morgan is unable to tell him specifically what he needs to do to engage his natural Traveler abilities, but does give him a useful hint: He might need a focus aid or mnemonic to help direct his concentration, kind of his personal version of a magic spell.
Trav realizes that music has always helped him focus. With this knowledge, he is able to successfully shift back to Stream One, but too late to prevent the murder of the Sam who inhabits that stream. Trav resolves to complete his interrupted journey to Stream Zero and eliminate the threat of Trav Zero once and for all.
On Stream Zero, Trav is able to turn the tables on his rival, but is shocked to discover that Trav Zero is much more concerned with saving the life of Kaaro’s informant, Rob Lennox, captured but still alive on this stream, than he is in bargaining for his own. Zero tells Trav he will answer all of Trav’s questions, but only after they rescue Lennox. Reluctant, but curious over the behavior of someone who is not acting at all like a crazed killer, Trav agrees.
The Travs head to the abandoned warehouse where Lennox’s body was found on Trav’s home stream. There, Zero introduces Trav to yet another Trav Becker who has been acting as lookout. This Trav appeared on Zero’s doorstep, needing a place to recover from a wound suffered in a conflict he refuses to discuss. Trav dubs him Hurt Trav.
Trav asks his doppelgängers why they are going to all this trouble to save one person, a person who is already dead on at least one other stream. Zero replies simply that every life matters, further undermining what Trav believed about him up to now.
The two sneak into the warehouse and discover that the facility is a giant meth lab. Grymzin, Kaaro, and a bound, but still living Rob Lennox, are all there. So is Amy Harper, who to the astonishment of both Travs, has been working for Kaaro all along.
In the ensuring gun battle, Grymzin, Amy, and Zero are all shot. Kaaro mortally wounds Trav, before being killed himself by a barely-conscious Zero. Neither Trav is any condition to get them out of the now-burning building. But Zero has a very special Traveler trick up his sleeve. As Trav bleeds out, Zero summons up all the different versions of their battle, finally locating one where Trav’s wounds were minor. He blends that stream into theirs. Miraculously healed, Trav is able to get the two of them, plus Rob Lennox, to safety.
Just before the building explodes, Bilol Grymzin lurches out and takes aim at the Travs. But before he can fire, he himself is blown away by a hail of bullets fired by Morgan Foster and Mike Becker, Trav’s dad, alive on this stream.
Morgan puts a lip lock on the recovering Zero that shocks Trav as much as seeing his dad. Zero explains to Morgan with a chuckle that Trav is from the “Mary-verse.” He then offers to answer Trav’s questions, but Trav is pretty sure he finally understands what is going on. Trav shifts out before the authorities arrive, but not before having an emotional opportunity to talk to his dad one more time.
On Stream One, Sam Zero’s instruments show chaos on Stream Zero that he doesn’t understand until Trav shows up. Trav has realized that it has been Sam Zero, not Trav Zero, who was driven insane by his knowledge of the streams and who has been running from stream to stream. Unable to defeat Trav one-on-one, Sam has been recruiting Trav Beckers to do the dirty work for him, convincing them, as he had our Trav, that Zero was the true villain. With the help of Sam Four, Trav takes Sam Zero back to Stream Four. They arrange for Sam Zero to be discovered with the body of Trav Becker, guaranteeing that he’ll be put away for a long time.
But this means neither Trav nor Sam can return to Stream Four. Fortunately, Stream One now has vacancies for both of them. Sam is delighted to discover that his research here is much farther along. Adam is still alive here, Trav still has his job, and he also has Mary. After what the two of them have been through, it feels like a fair trade.
Trav and Sam have both seen enough multi-dimensional mojo to last a lifetime. Sam decides to take his research in a different direction, and Trav swears off using his Traveler powers. Well, almost. Seeing an alternate timeline where Adam finally asks Kim the waitress out, he nudges things in that happy direction.
But Trav is being watched. An unknown observer is disappointed that Trav has no intention of becoming a “quantum cop.” Another watcher, however, is convinced that Trav will be there when he is needed.