Like the band's preceding two albums, On the Threshold of a Dream follows a concept. It explores dreams, especially on the second side, which climaxes with the ""Voyage"" suite, inspired in part by Richard Strauss's Also sprach Zarathrustra. The piece, by Mike Pinder, features mellotron orchestration and flute.
The album begins with a poem accompanied by electronic sounds, and these sounds also appear at the close of the album – most vinyl pressings of the album continue the sounds into the album's run-out groove, causing them to play continuously until the record player's tonearm is lifted. Tape and CD album versions employ a slow fade.