On a barren landscape, an older man approaches a younger one and asks how it’s going along. The younger boy says that it’s going fine, and says that it only seats one.
At Mount Justice, Nightwing goes over the data file that Miss Martian and her uncle recovered on Malina Island. He informs Robin and Beast Boy that it shows that the Kroloteans were using abducted humans as guinea pigs to analyze something called the “meta-gene.” Before they can pursue the matter, the computer detects an incoming energy impulse. A ball of lightning forms and a metal capsule appears. The young man emerges, wearing a costume, and announces himself.
Nightwing orders the computer to shut down the cave, while the newcomer hears them discuss the energy impulse he caused and decides to call himself Impulse. He knows all of them by name and starts superspeeding around the cave, looking at everything. Nightwing tells Beast Boy and Robin to take “Impulse” down and they go after him. Impulse turns on the water in the shower, intrigued by the old-style faucets and turning on the water. Beast Boy and Robin manage to distract him long enough for Nightwing to stun and restrain him.
In Tibet, Red Arrow and Cheshire arrive at a temple in the mountains. They have their daughter Lian with them, and Cheshire tells her husband that he needs to get his guilt over the real Roy Harper’s disappearance resolved before he can be a father.
Impulse explains that he’s Bart Allen, a tourist from the future, and insists that he’s part of the heroic legacy. He informs them that he’s the grandson of Barry Allen, the Justice League’s Flash. They don’t believe him but Impulse says that he’s there to meet his ancestor. Nightwing offers him a glass of water and Impulse realizes that he’s trying to get a DNA sample. He calls Nightwing and the others by their civilian names and insists that he knows them because he’s from the future. However, Impulse refuses to discuss their future, warning that he could alter the timestream. Mal Duncan arrives via the gate and Bart frees himself at superspeed and runs off. While Beast Boy tries to fly off, Nightwing tells Robin that he put a subcutaneous tracker in the water. He confirms that Impulse is heading for Central City and makes a call to a friend.
In Central City, Barry Allen and his wife hold an anniversary celebration for Jay Garrick and his wife Joan. As Barry gets a call about Impulse, Impulse arrives, announces himself as Barry’s grandson, and gives him a hug. Barry insists that he doesn’t have kids, but Iris tells her husband that she just learned she’s pregnant. Impulse informs them that they’re having twins, much to their surprise, and then realizes that he’s said too much. Wally West arrives and warns Impulse about saying too much about the future. Iris gets a call and Impulse tells her not to answer it. However, she takes it and learns from her boss that a new superpowered villain is attacking the downtown area and wants the Flash.
Downtown, Neutron blasts an honorary statue of the Flash and vows to destroy the original as well.
Barry dons his Flash uniform but Impulse suggests that he let the League handle Neutron. The Flash refuses and leaves, and Impulse goes after him.
Neutron is destroying Central City and threatening the pedestrians. Flash arrives to save several of them and goes after Neutron, who blasts him away. Impulse arrives in time to save his grandfather with a wind funnel. Flash warns him that Neutron is generating continuous energy waves, making it impossible for them to get to him.
At a nearby warehouse, the Kroloteans monitor Neutron and the results of their experiments with the meta-gene.
In Tibet, Cheshire and Red Arrow take out the guards and descend into the temple. Once inside they subdue more guards but are soon outnumbered and captured.
Kid Flash arrives to help Flash and Impulse, and they tell the police to stay back. Flash suggests that all three of them generate a wind funnel and pull Neutron up into the atmosphere. They go after Neutron and start circling him at superspeed, pulling him up into the sky. He refuses to let them defeat him and generates ever-greater waves of energy.
The Kroloteans realize that their experimental subject is generating energy faster than he can control it.
Neutron overloads, devastating much of the downtown area. The faster Flash and Impulse carry Kid Flash out of the zone of destruction and then Bart starts to go back. Flash stops him while Kid Flash scans the area and watches as Neutron recreates his body. However, he has no idea what has happened to him or where he is The Kroloteans realize that the explosion destroyed their control device and abort the experiment, destroying the warehouse.
Kid Flash realizes that Neutron was wearing a containment suit and that without it, he will explode again, bigger than before. Flash realizes that there are now microsecond fluctuations in his energy waves and says that he’ll run between them at near-lightspeed and take Neutron to the desert where he will detonate harmlessly. Impulse tries to warn him against it but Flash speeds off, and Impulse follows and seemingly trips, knocking his grandfather down. Neutron detonates and Kid Flash and Jay Garrick, the original Flash, get them to safety just in time. Kid Flash accuses Impulse of endangering the Flash, but Flash admits that he couldn’t have succeeded. While they talk, Impulse slips away and goes to the blast crater. As Neutron starts to reform, Impulse attaches a small unit to him, neutralizing his powers. The others wonder what happened and Impulse says that Neutron must have run out of energy.
Red Arrow and Cheshire defeat their captors and enter a room holding the cryogenic chamber containing Roy Harper. They free him and discover that his right arm is missing.
At Mount Justice, Nightwing confirms that Impulse’s DNA matches Barry and Iris’. He also has radiation confirming that he’s a time traveler. Impulse says that he has to run and Barry thanks him for coming. They exchange hugs and Impulse tries to leave in his time-travel capsule. However, it fails to activate and he realizes that it doesn’t work. Without any way to fix it, he’s trapped.
On a barren landscape in the ruins of Mount Justice, 40 years later, the older Neutron approaches Bart and asks how it’s going along. Bart says that it’s going fine, and says that it only seats one. The older man warns that the time capsule will burn itself out and Bart will have no way back. Bart says that it’s better than being stuck with the future they have. Neutron gives Bart the unit to depower him and warns that curing him and saving the Flash is only the beginning. Bart agrees and departs in the time capsule, and Neutron realizes that he’s changed. However, the rest of the future is the same, and he realizes that Bart failed in his primary mission to alter the past.