From the Super Mario Wiki
Enter Bowser?! (also referred to as Challengers Episode 3 and Challengers 3) is the third 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 Wind Master, and unlocks Mario and Sonic once it and The Mysterious Thief are completed. 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.
Yoshi, Sonic, Tails, Shadow, Silver and Jet turn towards the source of the laughter, which turns out to be Bowser Jr. rather than Bowser as they had previously thought. Tails asks Bowser Jr. how he managed to make his voice sound like Bowser's, and he explains that when he breathes in the Phantasmal Fog it allows him to sound like Bowser. Bowser Jr. asks if he scared them, though Yoshi and Silver say that they were not scared, angering Bowser Jr. Shadow asks if he has anything to do with the Fog machine, and he accidentally tells them that it is Bowser's work. After Silver and Tails call him a little kid, Bowser Jr. challenges them to an Olympic event, promising to tell them where Bowser is if they win. Yoshi, Shadow and Silver accept the challenge.
After beating Bowser Jr., Shadow asks where Bowser is, though Tails suggests that they should stop the fog machine first. Shadow destroys the fog machine, causing the fog to clear up and allowing the bridge to be lowered again. Tails asks Bowser Jr. if Bowser is trying to stop the Olympic games, and Bowser Jr. says that he doesn't know, but mentions that Bowser has been discussing things with Dr. Eggman. Bowser Jr. realises what he said, and escapes on a Bullet Bill, and Yoshi chases after him. After gaining no information from Bowser Jr., Jet suggests that they investigate other foggy places around London, pointing out that there is fog around the British Museum. The group decides to split up, with Sonic and Tails heading to the British Museum.