That Time I Got Reincarnated as a Slime the Movie: Tears of the Azure Sea

In the underwater Kingdom of Kaien, a woman named Yura steals a magical flute called the Dragon's Fang and escapes from the kingdom.

A month after the conclusion of the Tempest Founder's Festival, Rimuru Tempest and his companions accept an invitation from Elmesia El-Ru Sarion, the Celestial Emperor of the Sorcerous Dynasty Thalion, for a vacation at a private resort. Rimuru also learns of recent nuclear magic attacks targeting Leon Cromwell's domain, who constantly deflects them, resulting in them hitting Kaien instead, though the kingdom survives thanks to a force field. Yura eventually arrives at the island and meets Gobta. The two fight off men led by Zodon, who was sent by Djeese, a minster from Kaien, to get the Dragon's Fang back. They manage to get away, but lose the flute in the process. After Zodon brings the Dragon's Fang to Djeese, he reveals his plan to awaken the mystical water dragon that protects the city to launch an attack on the surface world, using the flute to control it.

Rimuru and his friends meet Yura and learn about the ancient history of Kaien and the water dragon. Rimuru, along with half of his friends, set off to Kaien. Djeese and the water dragon ambush them along the way. During the fight, Djeese loses the flute, which Yura catches. Djeese is seemly eaten by the water dragon as it leaves. Yura heads back to Kaien to return the Dragon's Fang to its place while Rimuru, Shion, Ranga, and Gobta follow her. However, the flute breaks, causing the water dragon to disappear. Following a tour of the Kaien, Rimuru and his friends decide to stay for the night.

Djeese is revealed to have survived, but Zodon kills him, having been using him all along. Zodon then captures Yura and puts her in a trance, intending to marry her for his plan to conquer the world, revealing that Yura is a major part of the water dragon. The revived water dragon attacks the Resort Island and Kaien is brought to the surface. The dragon also summons clones of itself. As Rimuru and his friends fight the dragons, Gobta manages to free Yura from her trance. Rimuru eventually kills Zodon using a powerful attack from the sky.

The next day, Gobta confesses his love to Yura, but she reveals that she has no romantic interest in him at all before leaving, to Gobta's anger; the two remain friends though. Back in Kaien, Yura reveals that she doesn't just control the water dragon's core, she is the core herself who was reborn multiple times over the years and she dissolves into bubbles. Gobta hopes to see Yura again someday.

In the post credit scene, an egg washes to shore and the newborn baby water dragon hatches from it. Meanwhile, Diablo has defeated the primordial Jaune and learns that he works for Rimuru. Jaune takes an interest in Rimuru.