At Galaxy Police Headquarters, something has gone terribly wrong. The seal keeping Class A wanted criminal Kain locked up in subspace has failed, releasing him back into normal space. After engulfing HQ and turning it into a burnt out wreck, the powerful entity streaks towards Earth- with the only warnings of his presence a brief distress call, a mysterious aurora, and for Tenchi, a strange sensation as if he were about to disappear.
Some time later, however, all of that is forgotten, and Tenchi and the girls sit down to watch a home movie of his mother Achika on a high school field trip, as filmed by Nobuyuki. However, to everyone’s surprise, as they watch the movie, Achika begins to disappear, and in the present, so does Tenchi. Fortunately, Washu is on hand with a special time shield and energy net that protects Tenchi, but this is only a stopgap solution to a much bigger problem. Somewhere in the past, something has happened to Tenchi’s mother, changing the timeline so that Tenchi and even the Masaki house itself never existed.
There is only thing the gang can do to set things right- go back into the past and protect Achika from whatever danger is to befall her. After collecting Kiyone and Mihoshi from their apartment, Tenchi and the girls (minus Washu) take a somewhat uncomfortable ride back to 1970 courtesy of Washu’s chronometric regulator. As per their brief to look out for Achika, Ayeka and Ryoko join her school as transfer students, whilst Mihoshi and Kiyone take on the roles of high school teacher and janitor respectively. Meanwhile, having been warned by Washu not to meet his parents for fear of causing a temporal paradox, Tenchi is forced to skulk in the bushes with Sasami and Ryo-Ohki.
Despite Ayeka and Ryoko’s natural tendency to disrupt classes by fighting over everything, the girls manage to integrate into the past reasonably well. Claiming to live too far away to commute, Ayeka and Ryoko even manage to become boarders in Achika and Katsuhito’s house. Everyone is well placed to keep an eye on Tenchi’s mother.
After doing some further research in the present, Washu contacts the gang and tells them to stay on the alert. Whatever is due to happen to Achika is most likely to occur within the next week- and this is also the maximum amount of time that her shield can protect Tenchi from disappearing.
A day and a half spent in the past reveals no sign of suspicious activity (although Ryoko has a few suspicions about a mysterious brown-haired student, she decides to keep them to herself), and at a quiet meeting in a storage shed, the gang wonders how to proceed. It soon becomes clear that the danger is far from dormant, however, when Tenchi starts to vanish once again, and even as the pain hits him, the gang realise something else- no one is keeping an eye on Achika.
The girls quickly rush to find Achika, but to their horror, she is nowhere to be found. Fortunately, she is in far from dangerous company- in fact, she is out in the woods talking to her classmate Nobuyuki. Nobuyuki has dreams of becoming an architect, and came outside to work on the design for his dream house. Joining him, Achika steals a look at his work, and even comes up with some suggestions of her own to improve on the design. And, after overcoming his embarrassment, Nobuyuki agrees that her ideas are actually pretty good. Finally catching up with them thanks to Ryo-Ohki’s tracking skills, the girls are relieved to see that Achika is fine- and that her relationship with Nobuyuki is progressing along the right lines. However, only Ryoko seems to suspect that someone else is also watching- the mysterious brown-haired student.
That evening, Tenchi and the others are back in their makeshift camp, when Kiyone is suddenly teleported away in the middle of taking a bath. The culprit is none other than Washu, who has brought Kiyone back to the present to check out some important information with her. The energy signature from Achika’s disappearance matches the one from the destruction of GP HQ, and a quick download from Yagame’s databanks reveals the connection- Class A wanted criminal Kain. A hundred years ago, Kain went on a galactic rampage that destroyed over a dozen planets, and only ended when the GP confined him in subspace. Now, Kain is free once more, and according to Washu’s research, is most likely to strike within the next three days- a time period that coincides with the high school class trip to Tokyo.
The next day, Tenchi and the girls accompany Achika’s class to Tokyo, and despite a train delay caused when a fight between Ryoko and Ayeka shorts out the power, everyone makes it with minimal damage. After a brief night’s sleep, it is time for the sightseeing to begin, and soon the class are enjoying all the attractions of Tokyo. Having brought along his camcorder, Nobuyuki takes the opportunity to film Achika at every turn, even ‘interviewing’ her about the trip and her plans for the future- at least until Ryoko and Ayeka tease the young couple about their obvious feelings for each other. Watching from afar, Tenchi becomes concerned- if the girls mess around too much, his parents may never even get together.
However, things soon take a more serious turn when Tenchi starts disappearing again. As Sasami protects him with the portable energy net, Ryoko senses something close by- the mysterious brown-haired student. Determined to discover his game, Ryoko tries to catch up with him, but he is too fast even for her, and she soon loses him.
Later, the class are visiting the Nakamise shopping arcade, when the brown-haired man shows up again close to Nobuyuki and Achika. As he departs, Ryoko follows, only to lose him once again. As she looks around for him, the brown-haired student steps out of the shadows and presses a gun into her back. As he correctly identifies her name and criminal record number, Ryoko realises that the man is from the GP, and he confirms his status as a GP special agent. He explains that he is after Kain, who came to this time in order to destroy those with Jurai blood. It was the Jurai emperor who helped to seal him away a hundred years ago, and now he wants to erase all traces of their line in revenge- starting with Achika.
Ryoko tries to surprise the GP operative with an attack, but he skilfully evades her. After removing his mask to reveal himself as one of the catlike Wau, he departs on his mission to find Kain.
That evening, Washu takes Kiyone into the present once again, before sending her back with some additional equipment and an explanation as to their next move. Jurai energy will be needed to seal Kain away once again, but unfortunately, no living person (not even Katsuhito) could generate enough without killing themselves. Instead, they will have to create Jurai power from the Earth itself, using special devices to draw it from the six Five-Colour Fudo shrines. According to Washu’s analysis, Kain’s attack will occur the next day during the school’s visit to Tokyo Tower, and it is here that the main booster must be placed to absorb the power from the shrines.
As the gang decide who should visit where, Tenchi decides that he must go to Tokyo Tower and protect his mother. Even as he decides this, however, he begins to disappear, but this time, he refuses the help of the portable net- he must steel himself to endure this pain if he wants to effectively protect Achika. But it is clear the temporal disturbances are getting worse- for now Achika is beginning to feel the pain as well.
The next day, the five girls head to the Five-Colour Fudo shrines to place Washu’s devices, whilst Tenchi accompanies Achika and her class to Tokyo Tower. Main booster in hand, he squeezes into the tower elevator with Achika and Nobuyuki, ready to set it up as soon as they reach the observation deck. Unfortunately, even as the elevator begins to ascend, Kain appears above the tower. Enveloping the tower with crackling energy, Kain smashes through the side of the elevator shaft, but even as he reaches Achika, he is deflected by a hail of laser blasts from above, courtesy of the GP operative. Distracted, Kain leaves to deal with this new annoyance, and Tenchi and his parents-to-be rush for the stairwell.
By the time they have made it to the observation deck, however, Kain has already dismembered the GP operative, and is back to search for Achika. After commanding Nobuyuki to position the main booster, Tenchi tries to ward off Kain using his Tenchi sword. He manages to ward him off for a little while, but is soon tossed aside. As she sees Tenchi on the brink of being destroyed, Achika calls out to Kain to stop, and as she draws his attention he senses her Jurai power. Tenchi quickly tells Nobuyuki to activate the booster, and as he does so, Kain is drawn inside. However, even as the criminal is sucked back into subspace, he ensnares Achika in an energy thread and tries to draw her in. As Achika is pulled away, Nobuyuki and Tenchi try to pull her back, but Kain is too strong. He manages to pull both Achika and Nobuyuki into subspace with them.
Whilst Nobuyuki and Achika find themselves trapped with Kain in a desolate subspace landscape, outside, Tenchi is in despair. As Washu does not know the exact subspace co-ordinates to which his parents were transported, getting them back will not be easy, but there might be a way to manage it. It is a long shot, but it may be possible for Washu to link Tenchi to his mother’s brain waves. Then, Ryoko can use what he sees to teleport herself, Tenchi and Ayeka into subspace and rescue Tenchi’s parents. Once they are free, Kiyone will use the Dimensional Cannon super weapon to shoot into subspace and destroy Kain once and for all. But they will have to be quick about it, because the shield protecting Tenchi can only last a few more minutes.
Once all the girls are in place, Ryoko, Ayeka and Tenchi change into battle outfit, and Tenchi dons the helmet that will enable him to connect to Achika’s thoughts. As the device activates, Tenchi finds himself seeing through Achika’s eyes, as she watches Kain brutally toss Nobuyuki aside. Breaking free of Kain’s grip, Achika runs to the fallen Nobuyuki. Seeing him lying there triggers the dormant power within her, and she too transforms into Juraian battle guise, granting her not only strength, but a sudden clear vision of the future, and of the son she and Nobuyuki will one day have- Tenchi.
Realising that he cannot destroy Achika in her current form, Kain decides to switch tactics, and attempts to absorb her and Nobuyuki instead. As he begins drawing them in, the couple are unable to break free, but fortunately, help is at hand. As they teleport into subspace, Tenchi breaks Kain’s concentration by slashing his face, whilst Ryoko and Ayeka catch Achika and Nobuyuki. Wounded and angry, Kain attacks Tenchi, who finds himself no match for the malevolent entity.
Tenchi is soon tossed aside, but as Kain begins firing electric bolts at the others, Achika grabs his sword, and moves to stand against Kain. Enhanced by her Jurai power, both the blade and hilt of the sword lengthen, creating a mighty weapon that is easily a match for Kain. Counterattacking, Achika stabs and slices through Kain, finally chopping him into two halves that rapidly disintegrate.
Time is running short, and the gang quickly move to depart the subspace realm. However, as they pass through the portal back to Tokyo, Kain reforms behind them, and grabs onto Achika’s hair. With one last desperate effort, Achika manages to break free, and everyone makes it through just as Kiyone fires the charged dimensional cannon. Ripping through into subspace, the cannon blast finally vaporises what remains of Kain.
As they reach the base of Tokyo Tower, the time has come for the gang to say goodbye to 1970. Whilst Ryoko carries the unconscious Tenchi, Ayeka moves to Achika and the also unconscious Nobuyuki to complete one last task- she must erase their memories of what happened. Achika acknowledges that this must be done, and asks Ayeka and Ryoko to look after her son- having glimpsed into the future, she knows that she will not always be there with him. As they make their final farewells, Achika hands back the Tenchi sword, and Washu finishes transporting everyone back to the present.
Back in the present, Tenchi cannot help musing on the remainder of his mother’s life, and how the loss of her Jurai powers after the battle with Kain may well have been what shortened her life. Nonetheless, she managed to have a number of good years with Nobuyuki, as they started a family and built their dream home together. And even though Achika is no longer with Tenchi, he still has the rest of his family, and the friends he cares for so deeply. If she were still alive, Achika could only be proud of her son.