The date of release says it all; Francis' return from a disastrous war leaves him scarred and disillusioned. The parallels with Vietnam are obvious, although not overdone; Donovan's music plants this squarely in the seventies. A series of (simply portayed) religious experiences changes Francis' life, and gradually that of his friends.
Shot in Northern Italy in the bright colours and "spit and sawdust" that are Zeffirelli's trademarks (as in his Jesus, and Romeo and Juliet, and parodied mercilessly in Jabberwocky and Life of Brian), the cast rise to the occasion to produce a romanticised version of Francis' early adult life, slightly over the top, perhaps, but coherent and dramatically plausible. If the costumes or some of the scenes are extravagant, they nonetheless portray the ethos of the central Middle Ages very well: small town rivalries, the increasingly uneasy relations between rich and poor, the lumbering ritual of a rapidly stagnating Christianity that the Franciscans did so much to revitalise. If it didn't look quite like that, it ought to have done.
I have loved the film since I first saw it and still find myself singing the music (failed hippy!).