It's the morning of her party, and the eponymous "Clarissa" (Vanessa Redgrave) is a little apprehensive. She's married to a politician (John Standing) who has jilted her for luncheon on this important day, so she sets off to buy some flowers then returns to find she has an unexpected visitor. "Peter" (Michael Kitchen) and she have some history, and as the day unfolds we learn a little of just how that played out thanks to some flashbacks with Alan Cox and Natascha McElhone as their younger selves. These depict the build up to decisions and choices that maybe one, or both, wish now had been…