King Louis finds the fake letters to his bride from Buckingham, and the proof is going to be if her Royal necklace is missing. He arranges a ball to take place in 5 days. The Queen's Lady in waiting, Constance, goes straight to D'Artagnan to ask for his help. He won't be rewarded for this deathly mission to break into the Tower of London and steal back the necklace, except to win the affection of his lady. He accepts. The Musketeers tag along too.
First they escape from Rochefort and make it to the docks. Somehow Rochefort is already there, waiting for them. Constance acts as a diversion to allow the Musketeers to get onto the first ship heading over to England, yet she is soon captured. On board they hatch a plan.
Meanwhile Milady is already with Buckingham and has warned him that the Musketeers are coming. She gets into a horse-drawn carriage and is whisked away.
In London, D'Artagnan disguises himself as a Beefeater and makes his way into the Tower - but is immediately captured. He soon realises that he was used as bait, as Athos rolls up outside the windows on a giant blimp, and starts using a rotating machine-gun style cannon to lay into the building. D'Artagnan jumps an Olympic distance of about 20 meters or so quite easily, and lands on the blimp's railings.
They turn the blimp around, and the attack seems completely pointless, as they know that Milady stole the necklace back. James Corden is the carriage driver, and locks Milady inside, releases the horses, and then the carriage is hauled up inside the airship.
Inside she pulls a pistol on Athos - but it wasn't loaded. They take the necklace. She shows them her letter from The Cardinal, before she is taken to the side of the blimp. Athos can't fire on her, so she instead dives overboard - surely to her death a few thousand feet below.
After some more in-movie map effects (probably some of the best bits about the movie, which is a bad sign), a larger airship flies alongside with Constance tied to the bow, and Rochefort behind the wheel. They fire on the smaller blimp. In an exchange, D'Artagnan changes sides with the necklace in return for Constance. Immediately the bad guys start to fire on the Musketeers. Luckily a storm is brewing and they hide in a storm cloud. Even more luckily, they don't even seem to hit any air turbulence.
Unluckily for the Cardinal's guards, Rochefort is struck by lightning (!) and stumbles a little - but surprisingly makes quite a quick recovery. D'Artagnan seizes the necklace and runs back through the blimp, as the Musketeers have risen above the enemy and start to fire down on them, destroying the balloon with flame-throwers. The larger blimp crash-lands into Notre Dame. D'Artagnan and Rochefort continue their fight outside on the Cathedral roof. Rochefort is the better fighter, but doesn't watch his adversary close enough before he steps in for the kill, and is stabbed through the chest.
The big party is about to begin. The smaller airship crashes onto the palace grounds - under the pretext that it is a gift for the King. The Musketeers disembark. The Cardinal is setup as the hero, somehow, against his wishes. The Queen shows up, wearing the necklace, and everyone is happy.
The cardinal offers the Musketeers a job, which they refuse. Then it's the usual "One for all and all for one" salute.
As it ends, Milady wakes up, soken wet, to Buckingham's face. The camera zooms back out to reveal Buckingham on a ship this time at the head of an Armada crossing the Channel ... and surrounded by a fleet of blimp-airships.