The Pirates of Penzance.

As a young child, Frederic had been apprenticed to a pirate by mistake when he should have been apprenticed to a pilot. Now, having reached his 21st year, Frederic's indentures are at last over and he happily leaves the service of the pirates. When Frederic meets the beautiful Mabel, one of the many daughters (or wards in Chancery) of Major-General Stanley, they fall in love and decide to marry. However, complications arise when the pirates decide to marry the rest of the Major-General's daughters, themselves - and Frederic's birthdate turns out to be not all it seems.