To the Ruins!
From the Super Mario Wiki, the Mario encyclopedia
To the Ruins! (also referred to as Heroes Episode 4 and Heroes 4) is the fourth Heroes 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 A Rainbow In Fog, and unlocks The Promise to Toad once it is completed. This episode's completion is required to unlock the Story Mode Cleared badge.
Mario, Luigi, Toad, Sonic, Tails and Omega arrive at Stonehenge, which is also covered in Phantasmal Fog, where they hear someone's voice and see Froggy flying through the air. The group follow the voice and find out that it is Magikoopa, causing Omega to remember that Magikoopa gave him something to transport to Dr. Eggman, though he does not remember exactly what it is and suggests that they ask Magikoopa instead. Omega and the group approach Magikoopa and ask him about the item, causing Magikoopa to realise that Omega's control chip is no longer working. He offers to answer Omega's question if he can beat him in an Olympic event, and Sonic steps up to the challenge in Omega's place.
After defeating Magikoopa, the group ask him for an answer, but he instead uses magic to fire Omega away. Omega hits Big, who explains that he is looking for Froggy. The group then notices Magikoopa talking to Lakitu, and mentions having to abandon the search for a lid before leaving. The group tries to catch them to try and gain some clues, however the collision with Big restores some more of Omega's memory and allows him to remember that it was the Fog urn that he transported to Dr. Eggman. He explains that the urn was sealed to keep the fog in, but the seal had to be broken at Stonehenge by Magikoopa using the site's energy before it could be used. The fog did not affect him as he was a robot, so he transported the urn to Big Ben where Bowser and Dr. Eggman used the fog amplifying system to create more of the fog. The group decides to head for Big Ben, but Omega helps Tails to find the lid of the fog urn so that they can seal it again. The group rush off leaving Big behind after Toad thanks him. Big then finds Froggy and the pair head to London after them.