Parallel dimension (Telepresence)

became stranded in a parallel dimension while "exploring possibilities". (AUDIO: Master of Worlds)

Mobile conversion units were machines used by the Cybermen for near instantaneous cyber-conversion of their subjects in this universe. (AUDIO: Code Silver)

When this dimension was invaded by Cybermen seeking to conquer the multiverse, the Master used the opportunity to obtain technology to repair his TARDIS and followed the Cybermen back to his home universe. (AUDIO: Master of Worlds)