The currently standing church was built in the 11th century. Its dazzling mosaics and frescoes arrived around 1312, courtesy of its patron Theodore Metochites. After everything went down between the Byzantines and the Ottomans, the church was turned into a mosque and the mosaics and frescoes were whitewashed until the artwork was restored starting in 1948. Today, the church is just a museum.
The church is square in shape, and the first thing you see when you walk in is the parecclesion, which used to hold tombs and is covered in frescoes mainly dealing with death and resurrection. (The third photo in the slide show is a fresco where Christ pulls Adam and Eve out of their tombs.) After you leave this area, it's all mosaics, which still cover almost all of the upper walls and ceilings until you get to the nave which is nothing but bare stone. (The nave has three mosaics but they're all on backings which makes me think they've just been placed there to give visitors something to look at.)
My favorite depiction was that of Jesus healing a leper, mostly because I admired how the artist had interspersed cream-colored and black tiles to make the leper appear afflicted all over his torso. (To be honest, it made me laugh.) Unfortunately though, it was located just inside the inner narthex and didn't show up well in the photo.
There's a slideshow here but it doesn't appear to well, appear, in Google Reader; I think you have to view it through the blog page.