An accident involving time travel strands someone in the past; they have to figure out how to get back to their own time. The accident can take at least three forms: * The traveler didn't mean to time travel at all, and has to figure out what was going on in the first place in order to replicate it. * The traveler breaks or loses a crucial component of the time machine in the journey, and has to find it or improvise a replacement. * The traveler accidentally screws up the past, and the time machine won't work properly until they Set Right What Once Went Wrong.