That title has structure protection & is the likely cause of your problem with playback. The protection isn't an issue with the latest couple of libdvdnav4 releases but it appears it is with the version provided in lucid.
Assuming your drive is /dev/dvd, if unsure you could run this & check the /dev/ links
sudo lshw -C disk
Look for the section *-cdrom, if it shows /dev/dvd then use below command, if it says /dev/dvd1 then adjust, you could also use the /dev/srX instead where X is usually 0 or 1
If you are in region 1 then the main movie is title 24 so this may work, -
vlc dvdsimple:///dev/dvd@24
command means - dvdsimple:// (no menus), /dev/dvd (device), @ title number
(starting in vlc-1.2 the @ is now a #)
Otherwise it's quite possible you can upgrade libdvdnav4 & libdvdread4 to newer versions, there are no real dependency issues. I don't have a lucid install to d. check but shouldn't be an issue.
If so you need to upgrade both packages at the same time, I'd use the 11.10 packages. If you wish to let me know, I'll edit in how.
EDIT: - realized I had a lucid iso so did a quick boot to live session to check out. You may not be able to play Thor with the above method though it could be handy for some other structure protected disks. I went ahead & upgraded the libdvdread/nav libs & you should then have no issue with Thor.
In some cases even with the newer libs you may have to resort to playing by going directly to the main movie title, Cars2 is an example of that, Thor should be fine as normal
To upgrade 10.04 to newer libdvdread/nav
Will use the 11.10 packages, one could use the 12.04 though 11.10 will suffice.
Open a terminal, run this command, then leave the terminal open
sudo apt-get remove libdvdread-dev; mkdir dvd_fix; cd dvd_fix
Then go to these pages, pick your arch, right click on the top listed mirror > save link as, save into dvd_fix folder (it's in your home dir.
http://packages.ubuntu.com/km/oneiric-updates/libdvdread4
http://packages.ubuntu.com/km/oneiric/libdvdnav4
Once both packages are downloaded & in the dvd_fix folder run this in the still open terminal
sudo dpkg -i *.deb
that will upgrade you & disk should play
If you need or think you may need the libdvdread/nav dev files for building related sources
We'll do this way to avoid possible broken packages - do AFTER the above is done
In your dvd_fix folder create a new folder named devs
Go to these 2 pages & download the -dev packages to the dvd_fix/devs folder like before
http://packages.ubuntu.com/oneiric-updates/libdvdread-dev
http://packages.ubuntu.com/oneiric/libdvdnav-dev
When or if you need the dev packages installed then just open that folder, right click on libdvdread-dev & install with gdebi. Then do the same for libdvdnav-dev, do libdvdread-dev first
Best Answer
Try pointing VLC to
/dev/sr0instead of/dev/dvdwhen you open a DVD (CTRL+D). Also, did you install libdvdcss? https://help.ubuntu.com/community/RestrictedFormats/PlayingDVDs