Film Synopsis idea

My idea for the film synopsis is that there would be two main characters who the film is centred around their lives ; male ( Sam ) and female ( Lilly ) who are a couple. They are coming back from a holiday abroad. On the way back they run into a young gentleman ( Mark ) who looked very friendly. The young man starts to converse with them asking personal questions and discover that they all live in the same neighbourhood. As time goes on he becomes closer and closer to them - going to their house , meeting their friends and family etc. Strange things begin to occur since that they day they have came met this man at the airport. The couple begin to catch on to what the man's intentions are & slowly starts to distance themselves from him. 

Lilly starts to get flash backs of all the strange things that he'd do or say every time he'd come to their house/get together. They knew deep down that he was a dodgy person but didn't really take no notice of it as they thought that that was just him as a person ; his personality. Every time they'd see Mark , they would try to avoid him as much as possible. Mark was no fool and knew what was going on. He vowed to take revenge for humiliating him infront of everyone.