From the back cover of the 1976 Methuen edition
Whale's Mouth was a planetary utopia for forty million Earth colonists-but none ever returned. It took only 15 minutes to get there by instant-teleportation, but it was strictly a one-way journey. If you wanted to return, it was always possible to go the long way round-18 years each way by conventional spacecraft. No one relished that, of course. Then one man decided to try it, and encountered some very powerful opposition.