After Poo joins you in Summers, you learn that Mr. Spoon in the Fourside Museum has made an amazing discovery.
Talk to Mr. Spoon
Go into the Fourside Museum and talk to the woman at the front desk. Pay to enter the museum. If you read the sign on the dinosaur exhibit, the Photo Man will take your picture.
Go to the far left of the main room and talk to Mr. Spoon, who is standing in front of the door. He asks you to give him Venus's autograph. She is the new performer at Topolla Theater.
Go to the Topolla Theater, buy a ticket, and watch the show. Then go to the far left of the theater, and talk to the guy standing in front of the dressing room door. He will let you in. Go through the door and talk to Venus to get her autograph.
If you have given any Sudden guts pills to Escargo Express, get them now. Buy some Teddy Bears. You should get Super Bombs as well, and some Big Bottle Rockets. You might want to Teleport to Threed and buy some Vials of Serum from the drug store there.
Give Mr. Spoon Venus's Autograph
Go back into the museum and talk to the woman at the front desk to pay to get in again. Go to the far left of the main room and use the Signed banana on Mr. Spoon. He will let you into the room behind him. There is a ladder down to the sewer there. If you are ready for a difficult dungeon, climb down the ladder.
Go through the Sewer
Because of the dangerous nature of this dungeon, you might want to go through the easy way, then go back and get the treasures later. The easy way is to go down a ladder into the water and keep going to the right until you see the sparkle of the Your Sanctuary boss, then climb up the ladder near the boss and go fight it. After you beat it, the monsters in the sewer will run away from you, so you can collect the treasures easily.
If you would rather get the treasures as you go, then start by going to the right from the ladder and going through the door. Check the trash can to get a Broken Iron. Then leave the room and go to the left until you find a trash can that contains a Croissant.
From there, go to the right until you reach a ladder down to the water. Climb down, and go to the right until you find a ladder out of the water. Climb up it, then go to the left and go through the door, then open the trash can there to get a Broken Spray Can.
Leave that room and go to the right, past the first ladder, and go down the second one into the water. Go to the right until you find another ladder. Climb up and go to the left, and open the trash can to get a Rust Promoter DX. Then go into the nearby room to find a Magic Butterfly. Leave and re-enter the Magic Butterfly room to heal everyone up.
Then leave the Magic Butterfly room and go to the right until you find another door. Go in and get the Broken Bazooka from the trash can. Then leave the room and go down the ladder into the water. Go to the right, and climb up the ladder to reach the sparkle. Talk to it to start the boss battle.
Plague Rat of Doom
The Plague Rat of Doom has a high Guts stat, giving it a 50% chance of doing a Smaaaaash attack on its turn. Don't wait to heal your party members.
You can try using Flash on the rat, because it has a 50% chance of working. The rat is weak to Fire, but you should use PSI Freeze γ if you have it. Use Big Bottle Rockets, Bombs, and your strongest PSI attacks. You might also want to use Offense Down on it, and use Shield β or Shield Ω if you have them.
You can also try using Sudden guts pills to boost your Guts stat, giving you a better chance of doing Smaaaash attacks against the rat.
After you defeat the rat, go up the ladder and approach Magnet Hill to learn the next Sound Stone Melody. Then open the nearby treasure chest to get the Carrot Key.
If you didn't get all of the treasures in the sewer, go back and get them now, because the monsters there will all run away from you now.