In the seaside village of Hinashi Town, Kai Ashimoto's grandfather is one of the locals who reinforces the legend that mermaids lurk near Merfolk Island and consume humans. Due to allegedly attracting merfolk and subsequently causing the death of the elder's mother, music is seldom discussed in the Ashimoto household. Even so, Kai finds solace in making looped beats on his laptop and uploading them to the internet.

Despite his reserved and glum demeanor, Kai is approached by Kunio and Yuuho Ebina, two cheerful classmates that happened upon one of his videos. The pair ask him to join their rock band but are met with apathy. Nevertheless, Kai agrees to accompany them in a practice session on Merfolk Island, where he soon witnesses something that he cannot explain.

Later that night, Kai is composing music in his room when he attracts a familiar entity—a mermaid. As she twirls and grins to the sound of his stereo, Kai discovers a number of aspects about the being who introduces herself as Lu: she possesses incredible water manipulation powers; she buds a pair of functioning legs with the sound of music; but most notably, she is a truly benevolent creature who simply wants people to like her. Fostering a dynamic friendship, Kai and Lu gradually come to express their genuine selves, testing the long-standing boundaries that have deafened the resounding harmony among the people of Hinashi Town.

In 2017, Yoake Tsugeru Lu no Uta became the first Japanese production in over 22 years to win the "Cristal for a Feature Film" award at the 27th Annecy International Animation Film Festival. In the same year, the film won the Silver Prize for Best Animated Feature at the 20th Fantasia International Film Festival. In 2018, the movie received the Ofuji Noburo Award at the 72nd Mainichi Film Awards and the Grand Prize at the 21st Japan Media Arts Festival.

Director Masaaki Yuasa has acknowledged that some elements of Yoake Tsugeru Lu no Uta were inspired by Hayao Miyazaki and his work, particularly from the 2008 film Gake no Ue no Ponyo as well as Isao Takahata's Panda Kopanda.

The film was released on Blu-ray and DVD on February 5, 2019 by Shout! Factory.