After being rescued from the arena, the ship Serious Sam is on is shot down by a hostile Laser Turret (originally scrapped) and crash-lands in the downtown section of Siriusopolis, which is flooded with Mental's forces, who are trying to defend against the Alliance's attack.
Sam fights his way through the downtown area, taking out a lot of Mental's forces along the way. Eventually, he reaches another trolly system, which takes him to the outskirts of the Mental Institution.
1. After going through a grassy area with two arcs and a medium-sized waterfall in it, you will be in a right-angle shaped room with a water fountain and grass in the center of it. Go to the right of the water fountain until you hit the wall. Look to your left, and you'll see a phone booth. Walk into it and use it to register the secret.