La historia trata sobre el asesino Beyond Birthday y cómo el súper detective L intenta resolver el caso de unos asesinatos, junto con Naomi Misora, una agente del FBI, que ayuda a la policía de Los Ángeles a resolver estos crímenes espeluznantes. Cuando "A" (Alexander) fue denominado como el primero en la línea de sucesión de L, éste no soportó la presión y se suicidó, dejando a Beyond Birthday como el próximo sucesor. Sin embargo, Beyond no quería ser considerado como un simple remplazo, por lo que escapó de la Wammy's House, un orfanato presidido por Watari, donde formaban y cuidaban a futuros genios. Sirve como precuela de la saga principal, narrada por Mihael Kheel (Mello). Todos los derechos a Tsugumi Ohba y respectivos participantes.

(Y/n) was raised inside Wammy's Orphanage in Winchester along with Mello, Matt and Near. Years had passed and one day when L's two successors learn of their predecessor's death, they go their separate ways to solve his final case and uncover his killer, Kira. Matt and (Y/n) drifted apart over the years, they were surprised when they received a message from their old friend to help him in his investigation on Kira, to reunite after 5 long years. Now the three childhood friends are back together, how will things play out for them? And will (Y/n)'s old feelings for Mello resurface to interfering with their investigation? [THIS IS A REWRITTEN VERSION OF HEARTBEAT] [DISCLAIMER FOR THE ORIGINAL ANIME/MANGA. NOTHING IS OWNED BY ME EXCEPT THIS STORY]