Well. are you saying the movie actually plays for 20 minutes?
If yes. Then I assume you have already checked all the Media options in your Veiwer. Also what veiwer are you using? Singularity probably won't work at all, so I assume you are using FireStorm, therefore make sure you have all media options checked.
Secondly, make sure you have enabled media on the land /Parcel and added the media URL to your Whitelist via About Land > Media Tab.
Thirdly make sure you have allowed SLPlugin.exe AND dullahan through your firewall.
You do not need to worry about Flash etc that is all old and out of date info.
Additionaly , account websites like Netfilx , Amazon Prime and Hulu won't work. I often watch movies inworld via sites such as:
[To see links please register here]
,
[To see links please register here]
.
Lastly you can stream movies into SL via VLC onto a Prim with a media face, simply by adding your desktop address as the whitelisted url.
Good luck and have fun