The hairy banjo player goes nuts. Thousands of feet leave the ground. It's as though the biggest Christmas party in the city is taking place in the O2. And we're only getting started.
Granted, the rambunctious Little Lion Man might have made for a storming closing number -- the perfect encore, perhaps.
But the strangest thing about Mumford & Sons (raggle-taggle mischief straight out of London, currently selling more albums in America than One Direction) is the way in which Marcus and Co operate.
Yep, the 25-year-old music maker and his buddies have somehow managed to make a living out of rewriting the same song over and over again.
It might even be an interesting experiment (or, at the very least, an entertaining drinking game) to play one Mumford & Sons album over another, taking note of every rehashed, banjo 'n' bass boogie and stop-start, stadium-prepared chorus along the way. There are a lot of them.
So, really, Mumford & Sons (it's just a name -- none of them are related) could open and close a gig with any number they like, just as long as it involves a bass drum. And most of them do.
In a live setting, Marcus is not only a frontman -- he's also a guitarist, an occasional drummer and an all-round good egg.
And that's the other thing about the Mumford gang. Musically, they have an awful habit of repeating themselves, but that doesn't take anything away from the fact that they're an extremely likeable live band. Skilful players, too, with an extraordinary passion for the noise they make.
Tonight, the O2 is covered in lights as a rainbow of rich colours and warm designs -- not to mention the inclusion of a brass section -- tops off an effortless transition to the arena stage.
How did they get here? Easy. The Cave, I Will Wait -- these songs all follow the same formula, but they were made for the big league.
Indeed, Marcus and his gang are now ahead of the competition, but it wouldn't hurt to try something new on the next album.