Corner the Koopa King!
From the Super Mario Wiki, the Mario encyclopedia
Corner the Koopa King! (also referred to as Wild Ones Episode 2 and Wild Ones 2) is the second Wild Ones 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 Secret of the Fog, and unlocks A Rainbow In Fog once it, Stop Dr. Eggman! and The Banana Incident are completed. This episode, along with the other Wild Ones and Tricksters episodes, must be completed to earn the Story Mode 3 badge, and its completion is also required to unlock the Story Mode Cleared badge.
Sonic arrives and is immediately attacked by Bowser, who he narrowly misses. Knuckles and Vector arrive, explaining to Sonic that they had been chasing Bowser on a tip from Amy, but lost him in the Phantasmal Fog. The group corner Bowser to prevent him running away, however he explains that he only came for Sonic. He then challenges Knuckles and Vector to an Olympic Event, which they both accept.
After Bowser is defeated, Sonic tells Knuckles to destroy the Fog machine. Bowser tells him not to, though Vector stops him and Knuckles breaks the machine by punching it. After destroying the machine, Knuckles tells Sonic to head back to the main stadium, while he and Knuckles plan to join him in a while alongside Bowser after they finish interrogating him about the situation. Sonic then heads off.