Fourteen-year-old piano prodigy, Kōsei Arima, becomes famous after winning several music competitions. When his mother Saki dies, Kōsei has a mental breakdown while performing at a piano recital; this results in him becoming unable to hear the sound of his piano, even though his hearing is otherwise unaffected.

Two years later, Kōsei has not touched the piano and views the world in monochrome. He does not focus on excelling in any activities and often spends time with his friends Tsubaki Sawabe and Ryōta Watari. Kōsei meets Kaori Miyazono, an audacious, free-spirited, fourteen-year-old violinist whose playing style reflects her manic personality. Kaori helps Kōsei return to playing the piano, and shows him his playing style can be free and groundbreaking. As Kaori continues to uplift Kōsei's spirits, he quickly realizes he loves her, although she seems to only be interested in Ryōta.

During a performance, Kaori, who later explains that she is anemic and needs routine testing, collapses and is hospitalized. She is later diagnosed with Aplastic Anemia. She invites Kōsei to play with her at a gala but she does not arrive. Her health deteriorates and she becomes dejected. Kōsei plays a duet with a friend, which motivates Kaori to attempt a risky and potentially deadly surgery so she may possibly play with Kosei once more. While playing in the finals of the Eastern Japan Piano Competition, Kōsei sees Kaori's spirit accompanying him and realizes she has died during the surgery.

At her funeral, Kaori's parents give Kōsei a letter from Kaori that reveals she was aware of her impending death and became more free-spirited, both as a person and in her music, so she would not take her regrets to Heaven. She confesses she had been in love with Kōsei's piano playing since watching him perform at a concert when she was five. This inspired her to play the violin so she could play with him one day. Kaori fabricated her feelings towards Ryōta so she could get closer to Kōsei without hurting Tsubaki, who harbored affection for Kōsei. She then confesses her love for him. Tsubaki comforts Kōsei and tells him she will be by his side. Kaori also leaves behind a picture of herself as a child, coming back from the concert that inspired her, with Kōsei in the background. Kōsei later frames this picture.
What is Your Lie in April about?
Your Lie in April is a manga and an anime about friendship through music. The protagonist Kosei Arima is a very talented piano prodigy who goes through a mental breakdown from the pressure of performing and becomes deaf to the sound of his own piano. The world becomes monotonous and depressing until he meets Kaori Miyazono, a free-spirited violinist who reminds him of how to play music freely again. However, Kaori suffers from anemia and often has to go to the hospital. Despite this, the two often play together and Kosei falls in love. Kaori passes away soon after and leaves a letter for Kosei explaining how she loved him back and his piano playing. Kosei uses this as inspiration to get back into music and play freely once more.