...could be if:
The Party could be journalists or archeologists and stranded long behind enemy lines when the twilight fell....I like the prison story too though...
They could also be :
Working for Red Cross on an humanitarian mission
They could be russian officials trying to defect
They could be a bunch of priests on a religious journey
They could be working for a huge english company/corporation (BP fex...)
They could be astronauts that got stranded in sibir because of a solar storm when entering earths athmosphere
The could work for a circus and touring in Russia
....only your imagination sets the limits here
