The Twelfth Doctor summoned his tenth incarnation to a planet, to instruct him on how to defeat the Vortex Butterfly. Having reinforced the temporal echoes to ensure his predecessor would retain helpful impressions, the Twelfth Doctor drew the answers out in chalk, with his explanations allowing the Tenth Doctor to realise that Gabby was at the heart of it all. When the Tenth Doctor left, the Twelfth Doctor gifted him the chalk as a way to help him remember. (COMIC: Vortex Butterflies)
