The debut album Broken Door was recorded by what was already the second incarnation of the five-piece band Prosper from the mining town of Bottrop in the north-western Ruhr region. Having previously split up, bassist Matthias Geisen decided to revive the band in early 1975. Together with guitarist Evert Brettschneider, he put together a new line-up for the group, now featuring the outstanding musicians Ernst Müller (Fender Rhodes piano, Mellotron, Mini-Moog, electronics), drummer Friedhelm Misiejuk and second guitarist, singer and lyricist Fritz A. Fey. That very same year, they recorded their self-produced debut LP at Blackfield Tonstudio in the Kirchhellen district of Bottrop, with Bernd Steinwedel as sound engineer.

Nobody knows why this talented band did not make any further recordings, as they played magnificent, complex and imaginative symphonic progressive rock with jazz influences. The eight compositions are sophisticated and stylistically diverse, featuring virtuoso guitar melodies by lead guitarist Evert Brettschneider. Their sole album was released in 1975 independently as a private pressing in a tiny run of just 500 copies. In 1976, Prosper disbanded once again by mutual consent. Soon after its release, their debut album was virtually impossible to find. Today it is regarded as a lost treasure of progressive rock and has disappeared entirely into private collections.