Trapped in an alternate timeline, Kid Flash is forced to compete in an archery contest, to win his freedom. Using his vibratory powers, Kid Flash speeds up his arrow. The shaft not only splits his opponent's arrow, lodged in the keyhole of a door, but also unlocks the door. Jupiterius, the alternate timeline version of the Teen Titan's mentor, Loren Jupiter, shows Kid Flash, and Mal, what waits for them behind the door. It is the ghost of the cro-magnon man that Kid Flash accidentally killed. The death of the cro-magnon man altered history, resulting in the strange medieval future Kid Flash, and Mal, are trapped in. The spectral appearance of the cro-magnon man fills Mal with terror. He bolts from the room, leaving Kid Flash to face the apparition alone. Mal races up to, and snatches the helmet of, the Duke of The Galaxies, the alternate timeline version of Superman.
Putting the helmet on, Mal charges the ghost of the cro-magnon man, which fades away. Mal has passed his test as well. A test of courage. Cerebella, the alternate timeline Lilith, had flooded Mal's mind with terror, to see if he could overcome it. Jupiterius leads Kid Flash, and Mal, to a wishing well. Dropping a pebble in the well, the two heroes bear witness to the events that created the alternate timeline. Mal had been hurled into the past by accidentally jamming Mister Jupiter's time machine. Kid Flash had run back through history to retrieve him. Kid Flash found Mal fighting a losing battle with a cro-magnon man. Mal had been edged off a cliff, and was hanging on for dear life, with the cro-magnon man closing on him. Kid Flash knocked the club from the cro-magnon's man's hand, accidentally causing the cro-magnon man to topple over the cliff.
Before Kid Flash could react, the club came down on Kid Flash, stunning him. When he recovered, he ran Mal back to the present, only to find it had all changed. Dropping a second pebble into the wall causes the water to crystallize. Kid Flash, and Mal, are instructed to consume the crystals. As the crystals dissolve in their mouths, they find themselves transported back to just before the crucial moment of the cro-magnon man's death. This time, Kid Flash knocks the club away, instead of up, then grapples with the cro-magnon man. As the crystals completely dissolve, Kid Flash, and Mal, are returned to the proper present day. With one hitch... the cro-magnon man came with them. Seizing Mal by the throat, the cro-magnon man is intent on killing Mal. It takes the combined forces of Kid Flash and Speedy to subdue the rampaging cro-magnon man.
Locking the cro-magnon man in an ape cage, Mister Jupiter turns his care over to the Teen Titans. It's up to them to civilize their new charge. Robin, the Teen Wonder, comes up with a lesson plan for educating the brutal savage. As it is all the cro-magnon man "says", the Teen Titans decide to name him "Gnarrk". Tranquilizing Gnarrk, Wonder Girl and Lilith cut his hair, shave him, and outfit him in modern attire. Gnarrk, upon awakening, is none to pleased with his new appearance. When Lilith comes too close to the bars of Gnarrk's cage, he seizes her by the throat. The Teen Titans are about to spring into action when Lilith stops them. Lilith finds she can communicate, telepathically, with Gnarrk. This revelation greatly speeds up Gnarrk's education. While Gnarrk sleeps, Robin attempts subliminal education techniques on him.
Upon awakening, however, Gnarrk is tormented by the newfound knowledge that his primitive mind cannot place into context. Lilith takes over the lion's share of Gnarrk's education. Soon Gnarrk is speaking English. The Teen Titans take Gnarrk out into the world. Terrified of the passing cars, Gnarrk attacks one. Robin gives the motorist money for the repairs, while Lilith explains cars to Gnaark. The teenage cro-magnon man is just about calmed down, when a passing train terrifies him into bolting. The Teen Titans scour the city for Gnarrk, but cannot find him. Lilith, though, homes in on Gnarrk's thoughts, and after soothing him, brings him back to their headquarters. Reading Gnarrk's mind, Lilith discovers that while he was hiding in the city, Gnarrk witnessed a criminal exchange between Councilman Buckminster, and a noted drug dealer.
From the information gleaned from Gnarrk's mind, the Teen Titans realize that Buckminster is the notorious "Santa Claus", the head of the Harlem's rackets. Realizing that they'll need Gnarrk to testify against Buckminster in a court of law, the team steps up his education. The added stress begins to wear on Gnarrk. It only gets worse when Buckminster's people throw dynamite, at Lilith and Gnarrk, on the street. To protect Gnarrk from further trauma, Lilith spirits him away, in the night. Buckminster's people, though, staked out the Teen Titan's headquarters, and thus were easily able to track Lilith and Gnarrk. The mobsters open fire on Lilith's van. Seeing Lilith injured, Gnarrk becomes enraged. He quickly beats down the two mobsters, then begins strangling the second man.
Having tracked Gnarrk by a transmitter Mister Jupiter placed under his skin, the Teen Titans arrive. Robin convinces Gnarrk not to murder the second gunman. Thanks to the efforts of the Teen Titans, Gnarrk has become a credible witness. Gnarrk's testimony sends Buckminster to prison. Lilith realizes she was wrong to try to shield Gnarrk from the modern world. Gnarrk's journey from savage to civilized man is complete. By choosing not to slay his enemy, Gnarrk has proved himself.