The alarm rings indicating that the gates to the alien prison have been opened.
Calvin and Susy get rather closer than they intend.
Seeing his chance, Spaceman Spiff makes a dash for a transport vessel, sneaks inside, and takes his seat.
Susy watches as Calvin runs into the bus as if he's being chased and calmly makes her own way into the bus and takes a nearby seat. It only makes sense given they'll be getting off at the same stop.