Complex sentence: The film takes place six years after the events in the first film which took place in 1940, showing Ralphie as a fifteen year old, who only wants a used 1939 Mercury Eight convertible for Christmas.
Split into simpler sentences: The film takes place six years after the events of the first film, which took place in 1940. Ralphie is now fifteen years old, and all he wants is a used 1939 Mercury Eight convertible for Christmas.