Sasami takes up the mantle of a magical girl and fights alongside Ryou-Ouki to help Tsunami become queen of a magical kingdom.
Sounds random, but if done right it works really well.
Sadly the first film is not done right.
Films 2 and 3 are actually pretty good, though the third one suffers from the same problems as the first.
The biggest issue I have is that too much time is spent on Ryoko and Ayeka chasing Tenchi, which doesn't add anything to the plot. On the contrary, it draws attention from it and makes it seem unfocused and all over the place. That probably also alienates viewers not
familiar with the main series (Tenchi Muyou!).
Since the first film has a similar plot to the 1996 TV show "Magical Project S" (which does it right), I recommend watching the series instead.
The other two films on the other hand are a must watch. Film 2 even starts off with a recap, making watching the first one unnecessary.