Like a Squidward Torture Porn, only with Plankton as the victim.
Any Spongebob episode where Plankton is the primary character is bound to be a Plankton Torture Porn.
While they are similar to Squidward Torture Porns, they also have differences. Both Squidward and Plankton are supposed to be "unlikeable" but are actually very popular, for example.
The main difference is that while SOME (very few) STP's are actually entertaining, PTP's are never funny. This is because they didn't start until AFTER the show had hit rock bottom (Seasons 6-7). "One Coarse Meal" is unquestionably the worst PTP and arguably the worst episode in the series alongside "A Pal For Gary".