The Blocked Bridge
From the Super Mario Wiki
The Blocked Bridge (also referred to as Challengers Episode 1 and Challengers 1) is the first Challengers episode in the Story Mode of the Nintendo 3DS version of Mario & Sonic at the London 2012 Olympic Games. The episode is unlocked after completing The Ghostly Tower and unlocks The Wind Master upon its completion. This episode, along with the other Challengers and Girls episodes, must be completed to earn the Story Mode 2 badge, and its completion is also required to unlock the Story Mode Cleared badge.
Sonic and Tails arrive at Tower Bridge and notice the Phantasmal Fog is also there, but is a different color to the fog at the Tower of London. Tails suggests that there may be another Fog machine around, and Sonic suggests that they go to find it. They head towards where the fog is thickest and meet Yoshi who is glad to see them. He explains that he cannot cross Tower Bridge because the bridge is lifted and cannot be lowered as the fog is too thick. Yoshi suggests that they try to talk to the bridge operator, when the group notice a pair of Fog Imposters of Shadow and Silver. They mistake them for being the real characters and ask them to help, but when they refuse to answer Yoshi, he challenges them to an Olympic event.
After defeating the Fog Imposters, Yoshi asks them to help again, though they both disappear. Sonic realises that they must be Fog Imposters, and Tails explains to Yoshi what they already know about them. Shadow overhears them talking and asks them for more information, and Silver arrives confirming that he is the real Shadow, as they had also dealt with some more Fog Imposters. Tails explains about the Boo using the Fog machine to produce Phantasmal Fog, and suggests that they search for another Fog Machine. Jet rides past on his hover board, causing Sonic, Shadow and Silver to chase after him. Yoshi and Tails decide to follow them, despite not being entirely sure what is going one.