In the episode Carl does Porter's shift in the cafe as favour so that Porter can go to Jupitar to see a once in 500 year event. However its illegal for humans to enter Heaven without dying, so Porter sprays him to hide the fact he's human.
Carl becomes a popular and hard-working member of staff during his shift. Carl meets a young Angel in training Lucy who is struggling with his human test, an AIT is required to pass before their allowed to go to Earth.
Carl accidentally spills coffee on his arm which Lucy wipes away which wipes away the spray revealing the fact that he's human.