Unai Emery returns to Paris St-Germain to exorcise the personal demons of the so-called "La Remontada" while providing the X-factor in Aston Villa's attempt to reach a Champions League semi-final.
When the key elements of Villa's strategy to outflank this outstanding, emerging PSG side - who have already beaten Manchester City in the group stage and sent Liverpool out in the last 16 - are studied, eyes will turn as much to the master European strategist in their technical area as to those carrying out his instructions on the pitch.
And the narrative of what promises to be a thunderous night at Parc des Princes is held together by a selection of sub-plots, with Emery the central figure.