On the square there is a grand former theatre, now an exhibition space but with a fine cafe fronting the square. There is a gorgeous 15th century bell tower, and the street here has medieval arcaded shop fronts, a very pleasant way to spend a hot afternoon. Eventually you notice what is missing, of course, and this is the church.

Saint-Désiré is a good quarter of a mile to the west, in a rather less salubrious part of town, among the cheap clothes stores and electronic games shops. It is set hard against the busy road, traffic almost scraping the south and east walls, but with a pleasant park to the west.

To look at the building in its urban setting, you would not think that it contains a significant Romanesque church. After several fires, the tower and east end were rebuilt in the 17th century, and the south side has generally been reconstructed. You enter beneath the tower, but then step into a space which is wholly different in character from the one you expect to find.

As your eyes become accustomed to the gloom, you make out the high stone arcades, with the shallow clerestory above. Rich coloured glass all around creates spaces of different light in the primitive aisles, and beyond the crossing the church lifts into a later narrow sanctuary full of glitter and richness.

There was a church here from at least the 5th century, when it became the resting place for the remains of St Désiré, first Bishop of Besancon. Nothing survives of this building, although some parts of it were incorporated when the current crypt was built in the 11th century. This crypt was restored in the early 1990s, and is stunningly beautiful. It reminded me very much of the one at Vezelay. It is divided into three aisles by two ranges of columns, and the arcades lead your eye to the east end and the coffin of Saint-Désire. Above the coffin, a slit window lets in golden light. Here, pilgrims could touch the bones, but the bones have been gone since 1794.

The great church above is probably contemporary with the crypt, although the east end has been largely refashioned. It seemed curious to step out of the brightly lit crypt into almost complete darkness, but as my eyes became accustomed again I gazed up at the soaring vauting. A coolness and a dampness struck me that spoke of age, but probably can't be terribly good for the furnishings. The two aisles end in baroque awfulness, the kind of kitsch cleared out of virtually every church in the area in recent decades. I dare say that there is every chance the whole church will receive the heritage makeover that the crypt got before too long, and I can't say I think this is a bad thing.

Saint-Désiré, Lons-le-Saunier, is to the west of the town centre, clearly signposted from the main square. Park in the town centre before looking for it.