From the Super Mario Wiki, the Mario encyclopedia
Mario's Defeat?! (also referred to as Wild Ones Bonus 3 and Wild Ones +3) is the third Wild Ones bonus 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 Life of a Minion, and unlocks An Innocent Request once it and The Fog Returns are completed.
Bowser and Goomba arrive at the London Tennis Club, where they meet Boo. Bowser asks why they have brought him to an empty court, and Goomba and Boo head off to sort out the issue. Goomba asks why Boo has not managed to bring Mario there yet, and when Boo explains that he could not, Goomba suggests that Magikoopa could transform him into Mario. Magikoopa, Lakitu and Dry Bones arrive, and Goomba and Boo explain the situation to them. Dry Bones heads off to distract Bowser, and Magikoopa agrees that they should transform someone to look like Mario. Goomba suggests that they should transform Boo, though Lakitu notes that Boo would float, so they decide to transform Goomba instead, ruling out Dry Bones as he cannot talk normally. Goomba offers to fetch Shy Guy, and explains to Bowser that Mario will be late, though Bowser becomes impatient and Goomba is reluctantly transformed whiled Dry Bones distracts Bowser. "Mario" arrives, and although scared, accepts Bowser's challenge.
Bowser manages to defeat "Mario", however doing so causes the spell to wear off and him to transform back into Goomba. Dry Bones distracts him as this happens, though Bowser is suspicious of the group as they encourage him to celebrate his victory. Bowser Jr. arrives and tells Bowser that he saw him beat Mario and was impressed with Bowser's performance, and the pair begin to discuss the match. Magikoopa, Boo, Lakitu, Goomba and Dry Bones notice that Bowser is back to his usual self, and wonder if it was Bowser Jr.'s absence causing him to worry. Bowser notices them celebrating, and thinks to himself that he is glad that they are happy, as he had been trying to cover up a particularly bad toothache by pretending he was fine. Bowser Jr. then asks him if he is alright, and Bowser tells him that he is.