11th- or 12th-century fresco on the left (south) wall of the lower church. It depicts St. Clement at Mass, as part of the story of Sisinnius told in the Golden Legend. Sisinnius was an unbeliever with a Christian wife, Theodora. One day he followed her to church to see what the fuss is all about, but after hearing a prayer by Pope St. Clement he was struck blind and deaf. He made it home with the help of his servants while Theodora told Clement what had happened. He came to Sisinnius and prayed for him, upon which his sight and hearing were restored. But the healed man suspected Clement of trickery in order to take his wife, so he commanded his servants to bind and arrest the saint. They tried to do so, but ended up binding columns and stones laying on the ground. Clement said to Sisinnius, "To things that are stones you give the name of gods, so you deserve to drag stones!^" In the end, Sisinnius came around and was baptized by Saint Clement.
The painted inscription across the middle of the fresco is the humorously vulgar command spoken by Sissinius to his servants: "Go on, you sons of whores! Pull, pull away, Gosmari and Albertel. You, Carvoncelle, get behind with a lever!" This epigraph is interesting not only for its content, but also for its language: it is one of the earliest known examples of the transition from Latin to Italian.