"Urusei Yatsura: Lum The Forever" is a 1986 anime film that serves as a continuation of the "Urusei Yatsura" series, created by Rumiko Takahashi. The story revolves around the residents of Tomobiki Town, particularly Ataru Moroboshi and Lum, an alien princess from the planet Oniboshi. The narrative begins with the town preparing for its annual Cherry Blossom Festival, but an unusual phenomenon occurs: the cherry blossoms bloom out of season, and Lum begins to experience strange visions and memory lapses. These events are tied to the discovery of an ancient tree buried beneath the town, which is revealed to be a remnant of Lum's home planet.

As the story progresses, it becomes clear that the tree is exerting a mysterious influence over Lum, causing her to lose her connection to Earth and her memories of Ataru. The tree, known as the "Tree of Life," is a sentient entity that seeks to reclaim Lum, believing her to be a part of its essence. Ataru, along with his friends Shinobu, Mendou, and others, attempts to save Lum by confronting the tree's power. The film explores themes of identity, belonging, and the bonds that tie individuals to their homes and loved ones.

The climax involves a dramatic confrontation between Ataru and the Tree of Life, where Ataru's determination to save Lum is tested. The resolution hinges on the strength of their relationship and the sacrifices made to preserve it. The film concludes with the town returning to normalcy, and Lum's memories and connection to Ataru restored, though the experience leaves a lasting impact on the characters.

The main characters include Ataru Moroboshi, a perpetually unlucky and flirtatious high school student; Lum, the alien princess who is deeply in love with Ataru; Shinobu Miyake, Ataru's childhood friend; and Shutaro Mendou, a wealthy and arrogant classmate. The film maintains the comedic and fantastical elements of the series while delving into more emotional and introspective territory, making it a notable entry in the "Urusei Yatsura" franchise.