This is one of those Gialli that doesn't have a masked killer slaughtering the cast, but still has plenty of the other elements still intact. It's similar to later films like The Designated Victim and Umberto Lenzi's Oasis of Fear as we're introduced to a limited amount of characters, and are left to figure stuff out as we know for sure something sinister is going on.
Even though this is a mid-sixties Italian film the setting is not an old castle in Scotland or wherever, but an old mansion on the Amalfi Coast! An English archivist and his young assistant arrive at the mansion of Count Paolo Barbarelli to archive his stuff (I guess). Next you know Paolo is giving Karen the assistant the glad eye and making his guard dog eat her shoes so he can buy her a new pair.
Karen wonders who the young lady wandering around the mansion is and Paolo explains that this is just his mentally ill daughter Cora, whom the archivist starts calling 'Lady MacBeth' (he's quite funny this guy). Next up Paolo dumps his more mature mistress and starts putting the moves on Karen. If she thinks that being a stepmother to a girl on meds is going to be tough, she's underestimating the circumstances.
I won't go much further with the plot but the whole film starts out like a romantic comedy, starts developing a bit of mystery, and by the last third is wearing it's giallo influence on it's sleeve, what with the pictures that hold clues and the twists and possibly even murder maybe. It's one of those films that gets better as it goes on, so if you stick around a bit it might pay off for you.
Or not. How am I supposed to know? Jesus.