I really love the character of Serena, and I think Elizabeth Montgomery did a wonderful job playing the dual roles, but this episode just felt really flat to me as Samantha and Darrin encounter the famous Loch Ness Monster. Of course, as this is a magical show, the monster turns out to be a warlock and not a sea creature after all. Not only that, he lives with all the comforts of home!
This is all just a little bit much to swallow, particularly when the "monster" looks more like a child's blow-up swimming pool toy than anything even remotely scary but the show is a comedy and we can make allowances for such things.
Serena and the warlock don't end up getting along very well and she finds herself the recipient of a mermaid's tail. That part, at least, is very funny!