A rat problem is an adventure for the 5e, made for four 1st-level game characters(PC). The game characters that will manage to survive this adventure, by the end of it should be between the 2nd and 3rd level.
 
"A rat problem is an adventure set in a specific location, The Smoking Dragon Inn in the town of Summerton. There, the characters meet Dylan the innkeeper, who can give them information about Severos, but first, they must help him with a rat problem in his cellar."