Underground Power Plant

Walkthrough: After finding all four musicians, Pauline will need Mario to fix the Underground Power Plant. From where she stands, head outside and speak to a nearby character to learn of the way to the power plant lies with the manhole covers. After finding out this key piece of information, take a right and head all the way to the end of the street to the Outdoor Cafe. There will be a manhole cover by the shop, the same you can use Cappy to get inside of.

Once in the Power Plant, you will find that poisonous Piranha Plants are the reason to the Power Plant needing repairs. On the other other side of the Warp Pipe you'll come to will be a very large room with plenty of Piranha Plants. There will also be powered down platforms found here, the same that you can hit with Cappy to spin in place, thus allowing you to then cross. Continue moving until reaching a platform with another Piranha Plant and several Stingbys, the same with a secondary platform nearby the holds Purple Coins. You can also use Cappy to reach some more far-off Purple Coins. After obtaining all the coins, hit both Platforms to have the begin spinning so you can reach the far wall.

Defeat the Stingbys on the far wall, and then move left where you can activate the vertical platform by hitting the extending device. As it spins, hop on the platform and ride it to the top. On your way up, take note of a Power Moon you can obtain form another platform by going back the way you came. At the top of the vertical platform, enter the Warp Pipe at the very end. Here, you will again meet up with Pauline. You will then be thrust into defeating two Giant Piranha Plants. Use Cappy to first clean up the poisonous goop, and then use him to plug up each of plants. As he plugs them up, jump of their heads with Mario for them to be defeated. After both go down, you will earn another Power Moon.