Out of the Unknown - 3x04 - Beach Head- Followmy.tv

Giant multi-generation starship with a virtual city of people on board. The ship has been travelling for so long that the occupants have long forgotten the mission and the purpose. They have invented a cult like religion worshipping the tree the wind and the grass even although none of them have ever experienced any of those things. Eventually alarms sound one day. The ship has navigated itself to the destination and the ""leader"" is informed of the mission purpose. The occupants are reluctant to leave the ship, something that the mission inventors had predicted, so the ship was flooded with gas to force the occupants out onto their new planetary home.