The song is set in an unspecified time and place. A mysterious singer, after obtaining the "key called words," begins to tie them into a song: a lullaby she wanted others to listen to, with a distinctive "Lu Li La" pattern. She also takes a toy she had always wanted and throws it out a window.
The singer questions the wishes and desires of the humankind, and she tells them to "go to sleep." She expresses uncertainty about the song healing the heart of someone with a sin of desire. The singer mentions twelve songs that denote time with scattered keywords; as she searches for proof of her existence in "Pandora's box", she discovers she isn't anyone. Regardless, she declares that she will continue singing, believing that the memories and traumas will continue melting and blending within her.
Re_birthday's PV mirrors Clockwork Lullaby, depicting its singer in a similar pose opposite of Clockwork Lullaby's mysterious singer.
The last line of Wordplay, "Teach me words as always", is repeated as the first line of the song.
This is the first song to set the distinctive "lu li la" tune heard in the series' succeeding songs.
The song was remastered for a second time, titled "Clockwork Lullaby -Key of Word-", featuring new lyrics in the bridge instead of an instrumental bridge; it was included in the Clockwork Lullaby album.