We find ourselves in Act One on a rocky seashore on the coast of Cornwall. The Pirates have just arrived and Frederic, about to complete his indentures as a Pirate apprentice, decides to leave the band and tells them he will, on becoming respectable, have no alternative but to betray them to the authorities. Frederic's nurse, Ruth, had mistakenly enrolled him as a Pirate when he should have been apprenticed as a Pilot!
Leaving the band Frederic stumbles across a group of maidens (with their chaperons) who are the daughters of a certain Major General Stanley. Just as Frederic begins to court Mabel, one of the daughters (it being love at first sight!), the Pirates and the Major General arrive. The Pirates, seeing the daughters and their chaperons, declare their desire to marry them - the Major General persuades them not to rob him, "an orphan boy", of his daughters, they being all he has in the world. The Pirates' better nature is touched. Only in an aside does the Major General confess he is no "orphan boy"!
In the Second Act we move to a ruined chapel, by moonlight, where the Major General is already regretting his "glib fib". Frederic, meanwhile, plans, with the help of the local constabulary, to capture the Pirates, but he is faced by the Pirate King and Ruth who explain to him that as he was born on the 29th February in a leap year, he is not 21 but in fact only 5 years old, and as his apprenticeship is until his 21st birthday and not for 21 years, is therefore still bound to them.
Frederic switches his "duty", back to the Pirates, bids a tearful farewell to Mabel and tells the Pirates that the Major General is not an orphan! Intent on revenge, the Pirates overcome the Police in a fight, but yield in Queen Victoria's name - even so, Will the Pirates get their man? Will Frederic get his Mabel? Will it resolve all happily?