Chelsea's recovery under Antonio Conte came to a crushing halt at the Emirates on Saturday evening.
The Blues went into the London derby unbeaten in five years against their rivals, but were swept aside at ease by the Gunners.
Alexis Sanchez opened the scoring after 10 minutes, before Theo Walcott doubled their lead in quick-fire time.
Mesut Ozil volleyed in a third just before half-time to leave the game out of reach for Conte's men.
Here are five things we learnt from the punishing defeat.
Chelsea's defence is a mess
Antonio Conte was left without a choice by the loss of both John Terry and Kurt Zouma through injury.
He was never going to try Marcos Alonso from minute zero at left back: as it would mean an untried option there, plus Azpilicueta in the recently unfamiliar territory of the right hand side.
But you could have placed bets on the Cahill-Ivanovic axis ending in calamity.
The bad news was this game came a week to early for Conte.
Were Chelsea at Hull this week and Arsenal next, the Alonso gamble may have been tried, and may even have worked.
The Premier League's fixture computer didn't allow that.
Hopefully, for Conte's sake, this is as bad as it will get.
Where is the N'Golo Kante of last year?
For Arsenal's third, it was Chelsea's dogged, terrier-like holding man who lost the ball to Mesut Ozil.
Nothing got past N'Golo Kante last season.
Here he watched it trickle away, before breaking into a genteel jog, and watching the ball go in the net.
This man was the best midfielder in England a year ago.
He's very far from it now, and Chelsea are really feeling that.
Conte's subs are still a mystery
No switches at half time – again – to the disbelief of most.
But the changes came, and much earlier than we've seen all season.
But taking off Cesc Fabregas , at Arsenal, to bring on Marcos Alonso was a curious one.
Chelsea strung five across the back, which created a Blue wall – at least until Alonso, still working has way back into English football, dropped a clanger moments later that earned him the ire, and a tongue-lashing, from Thibaut Courtois.
A very odd set of decisions.
Costa already heading for a ban
Diego Costa was one of the few who showed the passion expected in the shirt today.
But, in getting his fourth yellow of the season already – before September is up – he is only one card away from a ban.
At this rate he could miss a quarter of the season through yellow-card bans.
Play with passion, yes, but keep a lid on it, Diego.
There is no quick fix to these problems
Bad transfer window after bad transfer window. Repeated failures to replace key players. A lack of continuity and development in both managerial and playing circles.
One dramatic defeat at Arsenal does not a crisis make; but it does highlight a rot at the heart of the club that has been left to grow for some time.
The big issues here are less a case of whether Conte plays three at the back, or whether Ivanovic is a first team player.
They are more to do with where this club is heading, and how the squad is to be redeveloped.
This was a wake-up call. The question is: how many of those who matter are listening.