Many years ago, during the planet Jurai’s Startica summer festival, a little girl was eager to spend the evening with the planet’s handsome young prince. But that girl was no ordinary Juraian- she was a demon named Yuzuha, and when the Juraians realised who she was, they attacked her and chased her away.
It is an incident that Yuzuha has never forgotten, for even now, as she slumbers in her room full of toys and monitors, the events of that night still haunt her dreams. Awakening, Yuzuha decides to distract herself by checking out events on one of the monitors, which just happens to show the Masaki family enjoying Christmas. Recognising Tenchi as a Juraian prince, Yuzuha decides to have a little fun, and secretly plucks a few hairs from his head. After taking some hairs of her own, she heads down to a large bubbling vat, and tosses them in…
Some time later, summer has come to the Okayama mountains. Whilst the girls complete their usual chores around the house (with the exception of Ryoko, naturally), Tenchi finishes off cleaning up around the shrine’s cemetery before heading for home. However, as he heads down the stone steps, he passes a young girl sitting quietly on the ground. To Tenchi’s surprise, the girl greets him as “daddy”, and follows him home.
When Tenchi arrives home, the others are naturally curious as to who this new girl is, and matters aren’t improved when she insists that she is Tenchi’s daughter Mayuka. Nonetheless, they invite her in for a drink, but Ryoko’s temper soon snaps as Mayuka continues to refer to Tenchi as her father. Creating an energy sword, she goes on the attack, determined to force the girl to reveal her true story. Dodging, Mayuka manages to evade Ryoko’s attacks, but when the other woman corners her on the stairs, she suddenly creates an energy sword of her own and begins counterattacking.
Fortunately, Katsuhito arrives home just in time to put a stop to Mayuka’s attacks, much to the disappointment of Yuzuha, who has been observing the whole scene from afar. As Katsuhito suggests that Mayuka stay over for dinner, Washu surreptitiously plucks a strand of her hair for some DNA analysis.
As the whole family sits down for noodles, affairs are clearly strained. As Mayuka fawns over Tenchi, Ayeka and Ryoko’s tempers are clearly getting frayed, to the point where Ryoko can take no more. As she and Mayuka start fighting once again, Tenchi decides to escape on the pretext of wanting to go and take a bath. Unfortunately, Mayuka decides that she should come and bathe with him, and idea which is quickly squashed by Ayeka and Ryoko.
Later, the girls decide to question Mayuka a little more, determined to find out a little more about her back story. Mayuka admits that she doesn’t know anything about her mother or her past- the only thing she can remember is the face of her father, Tenchi. Emerging from her lab, Washu confirms that Mayuka is, in fact, telling the truth in this regard- according to the DNA analysis, Tenchi really is her father. Naturally, this seems more than a little odd given that Mayuka seems to be around Tenchi’s age, but given that there was a recent blip in the space-time continuum, it is entirely possible that she came from the future. In fact, one of the girls may even be her mother.
As the girls discuss this possibility, Mayuka quietly slips away- so that she can go and join Tenchi as he takes a bath in the floating hot springs. Tenchi is more than a little surprised to be joined in the bath by a naked Mayuka, but even as he tries to fend her off, Ryoko catches up with them. As she starts to chase after Mayuka, it looks like another fight is in the offing, but fortunately Katsuhito is on hand to break things up.
Still observing from afar, Yuzuha is once again annoyed at Katsuhito’s interference, but her feelings towards him are drawn from more than the present. He was the little boy she met at Jurai’s Startica Festival, the first person to reach out to the human side of a demon who previously enjoyed killing people and devouring their crystallised spirits. He even gave her a royal berry as a present, but, thanks to the Juraian men who attacked her, she never got to hand over the music box she wanted to give him in exchange.
Whilst Washu finds herself unable to use Mayuka’s DNA to identify her mother, Mihoshi, Sasami and Kiyone help Mayuka to wash her hair and change into a new summer dress. When Mihoshi spots a long silver hair on the brush she used to comb Mayuka’s hair, Washu quickly takes it for further analysis. Meanwhile, Mayuka decides to head upstairs to show Tenchi her new dress, only to discover that Ryoko and Ayeka are standing guard. As Mayuka heads back downstairs, Sasami suggests that she come and play instead, and soon she, Mayuka and Mihoshi are busy making origami cranes together (although in Mihoshi’s case the finished product turns into a pair of men rather than a crane). Delighted with her creation, Mayuka decides to make more cranes, and Sasami suggests that she could give one to Tenchi as a gift. This prompts Mayuka to remember that she already has a gift for Tenchi- a music box that opens up to reveal holographic images of Jurai’s summer Startica festival- a cross between Christmas and the Japanese Star Festival.
As the hot summer afternoon wears on, everyone soon dozes off, and Tenchi decides to take the chance to leave his room and quietly head off to do some work in the fields. Even as he goes outside, however, Mayuka’s music box activates, and Yuzuha sends a message to her to follow Tenchi. Acting under Yuzuha’s orders, Mayuka heads outside, and drags Tenchi off to the shrine steps on the pretext of having something to show him. Once they are halfway up the steps, Mayuka suddenly turns to Tenchi, telling him that she loves him. As if in a trance, she moves to kiss him, and the two are suddenly surrounded by a glowing green light.
Before matters can progress, however, Ryoko arrives on the scene. As the light dissipates, Ryoko grabs Mayuka, and flies off with her, determined to find out what was going on. When Mayuka cannot tell her anything, Ryoko flies out over the lake, before letting the other girl drop.
As evening draws in and Mayuka fails to return, everyone starts getting worried- all, that is, except Ryoko, who casually remarks that Mayuka was fine when she last saw her. Whilst Kiyone, Mihoshi, Nobuyuki and Katsuhito go out to look for Mayuka, the others start on dinner, but Tenchi finds himself unable to eat, and decides to go out searching instead. Fortunately, before he can leave, Katsuhito returns with Mayuka, who he found somehow stranded at the top of a tall cedar tree. As the others express their relief at her safe return, Mayuka rushes inside to slap Ryoko, shouting that she hates the other woman. Ryoko retorts that Mayuka is only getting in her way, and soon the two are fighting again.
As Tenchi moves to break up the fight, he too gets caught up in the heat of the moment, to the extent that he actually ends up slapping the hysterical Ryoko as she insists that he is in danger because of Mayuka. As Ryoko turns away in shock, Tenchi tries to apologise, explaining that all of the girls are important to him; unwilling to listen, Ryoko teleports away.
Later that evening, Ayeka finds Ryoko sulking out by the lake. The princess admits that she too is jealous, even though she swore it wouldn’t bother her if Tenchi ended up losing his heart to someone else. Ryoko tells her not to worry, before vanishing again as Sasami comes out to look for Ayeka.
Back at the house, everyone is having fun setting off firecrackers, and their festive attitude reminds Yuzuha of the hurt and prejudice she faced at the Startica festival. Inflamed with anger, she commands Mayuka to burn everything, and the girl obediently moves to grab a handful of fireworks. However, as the girls plunges all the fireworks into the flame of a burning candle, Ayeka quickly pulls her away, whilst Tenchi douses the fire with a bucket of water.
As they settle down for bed, Mayuka apologises for what happened, and admits she is scared of what she might do next time. To change the subject, talk turns to Christmas, and Mayuka soon finds herself entranced by the idea of this winter celebration. When she expresses a desire to experience Christmas herself one day, Ayeka invites her to stay at the Masaki house and celebrate with them as one of the family.
Meanwhile, Tenchi finally tracks down the missing Ryoko, who has been sitting by herself on the roof of the house. As Tenchi apologises for slapping her earlier, Ryoko explains that she is scared for his safety. Tenchi, however, does not seem concerned, and tells her not to worry so much.
Annoyed that Mayuka has been getting so friendly with the other girls, Yuzuha decides to step in, once again broadcasting orders via the music box. In a trance, Mayuka moves to obey and heads to Tenchi’s room. Climbing on top of Tenchi, she starts licking and kissing him, and once again, the green light surrounds them. Fortunately, she is interrupted once again- this time by the timely arrival of Washu and Katsuhito. Yuzuha is still watching through the music box, and Katsuhito immediately recognises her. As Yuzuha threatens to take Tenchi away, Katsuhito attacks her projection with an energy sword, and the demon beats a hasty retreat.
As the green glow dissipates once again, Ryoko arrives on the scene, and immediately grabs Mayuka to find out what is going on. When Katsuhito tells her to stop, Ryoko tosses the girl aside, and a scared and confused Mayuka runs away. Tenchi moves to follow, but Ryoko grabs him, begging him not to go and put himself in danger.
Whilst Mihoshi and Kiyone head out to search for Mayuka, Katsuhito explains to the others that the girl is under the control of the demon Yuzuha, a denizen of the Domain of Darkness. Ryoko is more than ready to accept that Mayuka should be treated as an enemy, but Sasami still wants to think of her as a friend, and runs out in the woods to help with the search.
As Mayuka runs through the woods, she begins to remember the things she did when Yuzuha was controlling her, and soon becomes distressed when she realises that her actions could have hurt Tenchi. Angry that her creation could have formed independent feelings for Tenchi, Yuzuha re-appears from the music box, and erases Mayuka’s memories. By the time Sasami catches up with her, Mayuka has reverted to being merely Yuzuha’s puppet. Taking Sasami hostage, she knocks down Ayeka when the princess arrives on the scene, and retreats to the World of Darkness.
Washu quickly opens a portal to the World of Darkness, and Tenchi and Ryoko prepare to head in to find and rescue Sasami. As they step through the portal, Katsuhito gives Tenchi a warning- once he is inside Yuzuha’s world, he will not be able to access his Jurai powers. The fighting will have to be left to Ryoko.
Entering the World of Darkness, Tenchi and Ryoko find themselves in a room full of animated soft toys- each bearing a deadly blade in its chest. As the toys launch an attack, Ryoko tries to fight them off, whilst Tenchi does his best to dodge all the flying blades. Finally, the attack lets up, but even as Tenchi and Ryoko plan their next move, the floor disappears from beneath them, and they drop down into Yuzuha’s next chamber.
Down in the main chamber, Yuzuha has put together her own twisted Christmas party, leaving Mayuka in the room to greet them. Ryoko immediately goes on the attack, but even as Mayuka is struck by a direct hit, she begins to transform into a powerful demonic form. As Ryoko and Mayuka continue their battle, Yuzuha arrives on the scene, and decides to liven up the party by filling the room with giant streamers that ensnare both Tenchi and Ryoko. Fortunately, Ryoko is able to break them free, and they beat a hasty retreat into another room even as another tangle of streamers comes after them.
Unfortunately, this new room proves to be even more hazardous, as Ryoko and Tenchi find themselves facing not only Yuzuha and Mayuka, but also an oversized teddy bear. Despite Ryoko’s best efforts to fight them off, the World of Darkness begins leeching away at her gem power, and soon she is too weak to fight back.
Tenchi tries to bargain with Yuzuha to let Ryoko and Sasami go free in exchange for his captivity, but Yuzuha is less interested in deals than she is in ‘playing’ with her prisoners. On Yuzuha’s orders, Mayuka begins slicing Tenchi’s skin with her claws, and as he cries out, Sasami calls out to her friend to stop. Somehow, Sasami’s pleas get through to Mayuka, reawakening the memories and feelings of friendship that Yuzuha tried to erase. Unfortunately, even as Mayuka breaks off her attack, Yuzuha decides that she has had enough of her creation’s disobedience, and envelopes her in a electrifying burst of purple energy.
As Mayuka goes back on the attack, it seems as if she has fallen back under Yuzuha’s control, but her target is not Tenchi and the others- it is Yuzuha. As Mayuka renounces the darkness once and for all, Yuzuha sends out another blast of energy. Falling to the ground, Mayuka manages one last goodbye for Tenchi and the others, before her body turns to pure energy. All that is left is a small red gem.
With Mayuka gone, Yuzuha lets go of the last of her human appearance, transforming into a full demon. As she enlarges her body to an enormous size, Ryoko tries to fly herself, Tenchi and Sasami out of harm’s way, but with her powers running out, she is unable to move fast enough, and soon all three are captured. However, even as Yuzuha holds them prisoner, Ayeka appears in the World of Darkness, bearing a branch from the holy tree Funaho. The light power from the tree branch is sufficient to let Tenchi transform into his Juraian battle guise, complete with Light Hawk sword. And, even as Yuzuha laughs at the puny size of his sword compared to her body, Tenchi launches an attack. And, as he slices her in two, he catches a glimpse of her past memories of Jurai- and of the young Yosho.
Having destroyed Yuzuha, Tenchi reawakens in the real world, with only Mayuka’s gem and Yuzuha’s music box as proof that the two ever existed. Tenchi hands the music box to Katsuhito, who shakes it open to reveal the royal berry he gave Yuzuha all those years ago. She truly did have feelings for him, and, in all honesty, young Yosho felt something for her too.
The summer days continue, but without Mayuka around, life at the Masaki house seems tinged with sadness. Nonetheless, the ever-brilliant Washu has a solution. She can use Mayuka’s red gem to resurrect the girl- but she won’t do it unless everyone agrees to the idea. All eyes turn to Ryoko to see how she will receive the idea, but to their surprise, she isn’t against it. Much as she seemed to hate Mayuka at first, the turning point was the day she dropped her from the sky- only to fly down and catch her before she fell into the lake. And now, with Ryoko’s go ahead, Washu is free to recreate Mayuka. She will have to be raised from babyhood, but at least this time she will be able to grow up from the beginning with a family that loves her.