Through The Looking Glass, as one guess, owes something to the works of Lewis Carroll, though more to Wonderland than the volume from which the title comes.
Star Catherine Burgess plays a w with a soap-opera existence who can't return her husband's affections. But there is an old mirror in the attic that she masturbates in front of, which to unleash her smoldering sexuality.
Seems there's a demon (Jamie G in mirror-land who's got the hots her, and as Burgess stares at her own reflection, the glass barrier opens (or does it?) to a series of carnal delights beginning with a girl/girler that segues into the Mad Hatter's tea party cum orgy.
Things get even stranger when Burgess sees herself as a young girl being seduced by Gillis as her father into giving him a blowjob. Burge finally accepts Gillis' entreats to join him on the other side of the glass, which leads to not a happy ending.
Along the way, we see (but not too explicitly) Burgess giving herself an enema; Terri Hall seducing her handyman brother; Kim Pope eating a banana out of Burgess' pussy, and more weirdness in mirror-land than even the '60s can be held responsible for.
An R-rated version of the film, it says here, had a fairly successful run on the drive-in circuit, but by today's standards, it's overly melodramatic and a bit light on decent sexual action. Still, retailers with a good number of classic film fans as customers will probably want to stock this 1976 production.