I remembered this movie with a lot of affection from when I was younger. "Chesser" (Charles Grodin), a diamond dealer operating on a small scale, and his girlfriend "Maren" (Candice Bergen) become involved in an elaborate scheme to inflate the value of diamonds—a plan that doesn't involve "Pussy Galore’s Flying Circus"—by diverting approximately $18 billion from worldwide reserves. The hub for this trade is a London townhouse, quite ordinary really, which serves as headquarters for the imposing Sir John Gielgud ("Meecham") and his loyal assistant "Watts" (James Mason). It doesn't take long to discover that “Watts” is living on borrowed time, and his employer isn’t particularly concerned about it—making him a likely candidate for collaboration with "Chesser." Their plan is clever, and the way they put it into action makes for an entertaining heist. Unfortunately, the film loses its way in the final thirty minutes, featuring too many abrupt conclusions, unnecessary complications, and a ridiculous horse chase involving Trevor Howard that turns it into just another standard crime story. I didn't really like the narration; it attempts to be wry, but it doesn’t quite land. It feels like a movie split into two distinct parts, with the first half containing all of its best qualities.

Source on TMDB