It was to take place in Germany but was aimed against Americans:

The Police have smashed a suspected al Qaeda terror cell nursing a “profound hatred of US citizens” plotting to bomb civilian and military jets.
The force of the planned explosions would have been worse than the train bombings in Madrid and the Tube and bus attacks in London on 7 July, 2005, according to German security sources. Those attacks killed 191 and 52 people respectively.
Three men aged 22, 28 and 29 have been arrested in Germany days before they planned to strike, and bomb-making equipment and explosives have been seized.
The arrests come a day after Danish police conducted raids and took eight young Muslims into custody whom they suspect of plotting a bomb attack and having links with al Qaeda. No direct link has yet been established between the two plots.
Federal prosecutor Monika Harms said the three suspects had bought 700kg (1,500lbs) of hydrogen peroxide to make massive bombs. She said: “We have prevented what we believe would have been the worst terror attacks ever on German soil”.
She declined to name specific targets but said the suspects had an eye on institutions and establishments frequented by Americans in Germany, including discos, pubs and airports.
[…]
The German intelligence service is said to have learned of the plot through emails.
The suspects are reported to have confessed. Security services are trawling their contacts to make sure there are no back-up teams.

(via)
They were also going to hit an airport.