I remember watching Saven Ultraman when I was a kid, and there was an episode on it that resembled the legend of Taro Urashima, and I'm curious to know which episode it was, does anyone know?

99OVA, Episode 4, The Result of a Promise.

This storyline is simple, but the clues are so messy that it gets tangled up in serious detail.

Briefly:

A strange old man suddenly appears on the beach in the coastal city of Ryugu. He's wearing outdated clothes, as if he doesn't belong in this era.

At the same time, the city of Ryugu began to intermittently experience time disorders, and the city returned to the desolate landscape of a few decades ago in a flash, and then changed back to the real world in the blink of an eye. In order to find out the truth, the Otter Police Force came to Ryugu City.

The Shima team meets a cold-looking girl on the beach and takes her back to a relative's house for careful care. Kazamori (Saifun), who is investigating on the streets, meets the delirious old man and takes the fainted man to the hospital. The doctor says that medically speaking, the old man is simply dead

Feng Sen is doubly impressed that something is wrong, and he notices a strange scar on the old man's right hand. Suddenly there was someone at the door, "Don't you worry about this man, Severn Ultraman." He claimed to be the man who connects two different worlds, connecting two different times.

"This is only the beginning, the past will flow into this place once more. By then, if this man still can't remember anything about the Promise, time in this place will completely collapse and disintegrate. You'll be as helpless as I am. Everything is caused by this man. It would be futile for you to try to help him."

The group asked what the "promise" he was talking about was. The middle-aged man turned and walked away, and the group chased him out of the hospital room, leaving the silent corridor empty. Rumi, who had come to deliver a snack, told Kazamori that she thought the incident was like Urashima Taro. In Ryugu-shi, there have always been stories about the Sea Dragon Palace: turtles carried Taro to the palace, where he was greeted by twin princesses; by the time Taro returned to the village again, 300 years had passed.

The ringing of the bells comes back to his ears, and the old man finally remembers everything:

Tai Lang has to go back to his village to report that he is safe, and then return to the Sea Dragon Palace to live with the princesses. The princess hands him a shell and instructs him never to open it until he returns to the Sea Dragon Palace. The princess used the sharp shell to leave the same scar on the palms of both men, and this was the pact between the princess and Taro. Taro promises to return, faster than a sunset flower can wither. Yes, the princess, who lived in the dragon palace under the sea, had never seen a beautiful sunset flower before.

The old man was the legendary Taro Urashima, who did not keep his promise and opened the shell as soon as he came ashore.

Taro apologized to the princess, but she said that no one would forgive him anymore. She holds up her right hand, and there is no scar in her palm. The one who made the promise with Taro was her twin sister, who died the moment Taro opened the shell.

Ehime threw the shell into the sea, and a large sea dragon appeared where the shell hit the water. Seeing this, the group immediately transformed into Saevon to stop the great sea dragon from taking revenge. The sea dragon had gathered the deep resentment of the Great Princess, which made Saiwen's fight hopeless, even the ice axe thrown was thrown aside.

Saven suddenly heard a voice asking, "Why do humans forget? Why do humans break promises? Why do you protect humans? Humans are such dishonest creatures."

The great sea dragon knocked Saiwen down heavily and made his way to Taro. Taro held up the long-faded flower in his hand and spoke, "This is the Sunset Flower."

Seeing the sunset flower the great sea dragon calms down and the wilted flower gets to bloom again. The great sea dragon sucks Taro into itself and then disappears into the air. Ehime also disappears into the waves.

The city of Ryugu is back to its usual calm, and the beach is still only filled with waves chasing and playing.