Poor Ian McShane - the best ever Heathcliff (BBC 1960s) and Judas Iscariot ("Jesus of Nazareth") reduced by 1979 to giving the only decent performance in this extraordinary mongrel of a film. As bizarre & characterless a quartet of musketeers as you can imagine, two porn stars playing important characters but seemingly only there to flaunt their bodies for no apparent reason, dear old Rex Harrison doing his best but clearly wondering what the hell he's doing there, the legendary Jack Cardiff clearly having an off day as cinematographer and a screenplay that takes the basic idea of "The Man in the Iron Mask" then hacks the whole story to pieces.
There's never been a screen version of Dumas' final Muketeers novel (except the presumedly lost 1968 BBC serialisation starring Roger Livesey, Jack Gwillim, Edwin Richfield and Noel Willman) that is faithful to the book, and this is no exception. The period detail's good, but otherwise the whole thing looks like one of those B-movie swashbucklers from the Fifties.
One day someone will make a decent version of this magnificent story. Until then you'd be advised to stick with stylish Louis Hayward version. Or read the book.