It was a puzzling display for Xabi Alonso to ponder after watching his team impose themselves on the hosts in the first half yet struggling to find a goal threat, before succumbing in the second period to goals from Jaidon Anthony, Igor Thiago and Fabio Carvalho.
This was a Chelsea line-up containing all the elements of their summer transfer re-think with a solid experienced spine which included Jordan Henderson and Danny Welbeck making first Premier League starts for the club.
Something appeared to have clicked. They did not look half as chaotic defensively in the first half as they have done at times this season, standing up physically to Brentford, winning duels and not allowing Keith Andrews' side to dominate second balls.
Yet old habits had followed the visitors on their short trip across west London. The marking from Mikkel Damsgaard's 61st-minute corner was poor, Jannik Schuster headed on and Anthony finished it off, then Thiago was left free to score after Kevin Schade's shot was saved.
It was then a low cross from the German which was whacked in during stoppage time by Carvalho to send Brentford fans into dreamland.
The first chance went to Chelsea and Welbeck showed clever feet and layed the ball off for Pedro Neto, whose drive met a strong save from Caoimhin Kelleher.
The absence of Joao Pedro was hard to ignore through injury and his replacement Welbeck was not quite so able to act as the middle link in a fluid trio with Cole Palmer and Morgan Rogers.
Both forwards were involved and looked dangerous, but they operated far less in combination than when Pedro is present.
All of Chelsea's good work defensively was undone just after the hour mark when Damsgaard's corner found not one but two Brentford players unmarked. Rising highest to head back across goal was Schuster and Anthony had the simple job of nodding in from virtually on the line.
Another lapse handed Brentford their second goal. Levi Colwill and Wesley Fofana were parted by a simple through-ball by Damsgaard, sending Schade clear. His shot was kept out by Martinez, but Thiago was there to knock in his first league goal of the season.
There was a final flourish for Brentford when Schade drilled low across goal in stoppage time and substitute Carvalho arrived to blast into the roof of the net.
The win moves Brentford up to third on the table.
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