Mary, Toadsworth, several colored Toads, and Peach go on a vacation to the fabulous Isle Delfino. As they arrive on Delfino Airstrip, they find the runway is covered with goopy slime-like graffiti! After further exploring, they find Delfino Plaza, along with many other areas of the island, are also covered with graffiti!
This goop sprouts Polluted Piranha Plants, along with little goopy monsters that leave goop trails. The goop has made the island dark, and Mary must clean the mess and retrieve Shine Sprites to make the island light again. Shadow Mary is revealed to be the culprit of this mess, but the Piantas and Nokis blame Mary of being the culprit, due to them looking so identical. Mary is now set out to clean up every acre of Delfino Island with the help of F.L.U.D.D., a device developed by Professor E. Gadd.
”Ugh, so much goop to clean up! I’ll make that shadow version of me pay for framing me for her mess!”