Otto Preminger's dark movie begins with a superb credits sequence from the great Saul Bass and thereafter follows the story of a missing girl who no one can recall existing. It's an existential dream like movie that takes the lead character, Ann Lake, played by Carol Lynley (looking remarkably like Lee Remick) on a journey into London's dark underbelly.
It's cast include Laurence Olivier and the little seen Keir Dullea (most famous for 2001: a space Odyssey) rounding out the cast are The Zombies pop group, with a publicity tie in, and Noel Coward.
Preminger himself wasn't overly fond of the movie but I think it has a weirdness that gets under the viewers skin and is well worth a viewing. Preferably at night with some strong coffee to hand.