Madrid have been in fine form of late, and have now won their last five games while scoring 20 goals in the process.
But their slow start to this Champions League campaign, in which they lost three of their first six matches, has ultimately cost them automatic progress to the round of 16.
Their vaunted attack in particular has found their touch too late - Rodrygo is now scoring at will, but only after going two and a half months without a goal.
Similarly Mbappe now seems to have adapted to his Real role as a central number nine, but the teething problems that were so evident earlier this campaign - such as the damaging loss at Liverpool - have still cost Madrid in the end.
It means Madrid must now go through an extra two play-off games in order to defend the title they regard as their birthright. While they will be well fancied to reach the last 16 and go further, it will be a sore point for Europe's most successful side that they have to take this road at all.
Brest, by comparison, will be thrilled to have reached the knockout stages of their first ever Champions League campaign.
The French side made a blistering start by going unbeaten in their first four league stage matches, but four winless games in the back half have cost them a top eight finish.
Eric Roy's side did have a tough draw - they are the first debutants to lose to Real Madrid and Barcelona in the same Champions League campaign since Leeds in 2000-01.