A 'mystery' play? Sounds as if it ought to mean a 'whodunnit' ...
In fact, the word comes from Medieval English meaning 'occupation, trade guild or company' and is possibly a corruption of 'ministerium' or 'the rendering of a service'. Once a year, local businesses used to work together on different Biblical scenes to produce a long pageant or 'cycle' - the major difference being that their performances took place outside.
Tonight is the culmination of 'services rendered' by many different groups and people in Eynsham and surrounding villages.