Cag was the captain of the transport vessel, the Argo Moon.
C-3PO and R2-D2 were traveling onboard the Argo Moon to meet their master, Mungo Baobab, on Biitu. Upon arrival at Biitu, Cag chided C-3PO for the mess he had made of Baobab's possessions. Before the ship could dock, it was attacked by droid minions of the Great Heep, and Cag and his copilot were rendered unconscious with sleeping gas. The droid minions abducted the droids, but left the organic crew.
After Baobab, Threepio and Artoo had defeated the Great Heep, Cag contacted them and made preparations for landing.