Plot Overview: Series One
The first series runs from the sinking of the Titanic (April 1912) to the start of WWI (August 1914). The central theme of the series is the matter of who will inherit the Downton estate and the title of Earl of Grantham. Only males can inherit, and the Earl and his wife have only daughters. They planned to get round this by marrying their eldest daughter Mary to the son of the next heir, but he and his father are killed in the sinking of the Titanic.
A distant middle class cousin, Matthew Crawley, becomes the heir apparent. This means that he will inherit both the title and the estate, which includes all the Countess’ fortune (the Earl married her for her money, and part of the marriage settlement was that her fortune will pass to whoever inherits the title).
The family hopes to marry Mary to Matthew, but she has a complex and scandal ridden love life which prevents her agreeing to this (Matthew is keen) until it is too late. The series ends with Matthew, disenchanted with the aristocratic family and their machinations, deciding to leave Downton for good as WWI breaks out.