The movie begins in the past on Planet Jurai. There is a celebration of the summer solstice called Startica. A young Yosho meets a strange looking creature. Their friendship is cut short by Jurain soldiers who drive away the creature. The creature wakes up from her memory and turns on one of the many TVs in her bedroom. Seeing the Masaki Christmas party, she decides to send a special surprise for Yosho. She steals a lock of Tenchi's hair and adds it to some of her own and throws it into a vat. The DNA combines and something is created.

Back at the house everyone is working hard on spring cleaning, except Ryoko. On Tenchi's way down the hill he meets a strange girl. The girl walk up and says, "Daddy." The girl claims to be Tenchi's daughter from the future. Her name is Mayuka. When Ryoko confronts Mayuka about her identity she panics and generates a Lighthawk Sword. Yosho breaks up the fight. And Washu steals a sample of Mayuka's hair for an experiment. At dinner Mayuka is making Ayeka and Ryoko very upset.

Later that day all the girls are talking to Mayuka. She seems to be suffering from a case of amnesia. Washu tells Tenchi that her tests have proven that she is his daughter. But she can't tell who the mother is. Washu suggests that one of the girls might be the mother. Mayuka shares qualities from all the girls. Its hard to tell who she's most like. Back in the creature's lair she is remembering just before she met Yosho. She'd just killed three of his friends bur he didn't know that. He offers her a berry which is very rare on Jurai.

Mihoshi and Kiyone help Mayuka get dressed. After Mihoshi is done brushing Mayuka's hair she discovers a silver strand which Washu gets to continue her experiment. While Mayuka, Sasami and Mihoshi are doing origami Mayuka remembers she brought something for her daddy. Its a music box which celebrates Startica. The creature appears in a hologram and Mayuka appears to be in a trance drags Tenchi to the Shrine. On the stairs she tells Tenchi that she loves him and kisses him. Tenchi begins to be transported away. Ryoko, Washu and Yosho all sense something is wrong. When Ryoko separates them she drags Mayuka away to extract some information. She takes her high above the trees and when Mayuka says she doesn't know what's going on, she drops her.

Later that night Mayuka hasn't returned they send out search parties. After its dark everyone returns home. Yosho finds Mayuka stuck in a tree and returns with her. When she gets back to the house she begins to fight with Ryoko. When Tenchi tries to break them up he slaps Ryoko without thinking. When he tries to apologize Ryoko leaves. That night Yosho visits Washu in her lab they discuss the silver hair. That night everyone is lighting firecrackers and Mayuka gets that look in her eye and under the orders of the creature takes all the firecrackers and puts them in the fire. Nobody is hurt by the resulting explosion. Later that night Sasami is sharing the photo album of last year's Christmas party. She's so impressed by the pictures that they invite Mayuka to join in next year's party.

That night Tenchi catches up with Ryoko and apologizes. After everyone is asleep the creature visits Mayuka again and order her to transport Tenchi to her dimension. The process seems to involve a lot of licking? Yosho and Ryoko interrupt. Yosho discovers that Yazuha is behind everything. Yazuha is a demon who is the essence of the dark tree of Jurai. Yazuha is the only evil Jurain tree I know of so far. Mayuka runs off that night and Yazuha takes her over again. This time she erases all Mayuka's memories. She becomes a mindless zombie. Mayuka kidnaps Sasami and takes her to the dark realm. Ryoko and Tenchi go off to save Sasami. Now I don't know why they sent Tenchi? They knew his Jurai powers wouldn't work in the dark realm but they sent him anyway. Now me I would've been smart about it. I would have sent Ryoko, Ryo-Ohki, Kiyone, Mihoshi, and especially Washu because their powers don't rely on Jurai power. Once inside the dark realm they find Mayuka. She turns herself into a creature more resembling Yazuha. In the dark realm they have to fight Mayuka, Yazuha, killer toys and lots of entangling wrapping paper. Its like the Christmas from hell. Yosho and Ayeka devise a scheme to help Tenchi fight while he's in the Nightmare World. They need the help of Tsunami, the Tree of Light, which also didn't appear in the TV series. But the closest thing they can come up with is Funaho, Yosho's tree. Mayuka begins to kill Tenchi but her memories surface and she can't do it. She attacks Yazuha and Yazuha kills her. She turns her back into the crystal from which she came from.

Tenchi gets pissed. Yazuha takes her real form. A small demon looking thing. And grows in size to a giant shadow. Ayeka shows up and throws a branch from the tree of light into the ground. Tenchi regains his powers. He creates the Lighthawk Sword and slays Yazuha. Yazuha says, "Imagine the darkness in love with the light." just before she dies. Tenchi wakes up outside the Nightmare world. He still has crystal which Mayuka turned into. Using the crystal Washu comes up with a way to revive Mayuka. She asks if anyone has anything against it. Nobody does not even Ryoko. The crystal grows into a baby girl and Mayuka gets to grow up.

After this movie there trailers for this movie before its release in Japan. Notice that they call it Eve of Summer.