From the Super Mario Wiki
Prior to the start of the level, the Koopalings regroup at Beanbean Fields, with no luck in finding Bowser. They are ambushed by Fawful, who quickly hypnotizes all seven of them. Meanwhile, Captain Goomba and his squad manage to arrive at Beanbean Fields and battle some Troopeas and Shy Guys. Afterwards, Captain Goomba comes across a Shy Guy who asks to join his squad as another captain after explaining that the captain is affected by the Power Triangle as well, of whom the former reluctantly accepts.