In T2 Trainspotting (2017), Simon (with his girlfriend Veronica) and Mark meet together and discuss about the past few days. Then suddenly Mark looks at his watch, which makes Simon very angry, and he wants to know why Mark is doing that. Mark replies that he has a plane to catch. And then Mark becomes even more angry, and leaves the place and takes cocaine himself. Now Mark starts talking with Veronica, and calls her plan B:
Mark: So you're Plan B.
What exactly is Mark referring to here by calling her plan B?