Bertrand was married to Alys. Therefore, Alys was married. What's the missing premise?
The problem states that : Bertrand was married to Alys. Therefore, Alys was married.
The premise here is: Bertrand was married to Alys.
The conclusion is: Alys was married.
The missing premise is: If a person is married to another, both of them are married to each other.
(It should be noted that reciprocity does not exist in all cases. For example if Jane is John's sister one cannot conclude that John is Jane's sister too though that would be the case if John is the name of a girl)