Michael Jackson and Tesco Bags

This joke plays on the dark rumor that Michael Jackson was a child abuser, and makes a comparison with plastic bags. It's unexpected to find such a link between such a controversial person and an everyday object. The dark humor comes from the shock of making light of the rumors surrounding Jackson, who is very famous and was often seen as a child-like character. The second part of the punchline is meant to be shocking given the context of Jackson's history.