Bio Senshi Dan is played across five areas. In order to complete the game, you must defeat the bosses of all five areas. The boss of each area must be found and defeated in order to advance to the next area. Each area contains a number of open spaces and hallways which are each connected via passage ways and elevators. Dan must typically jump into a passage way, which usually resembles a small window or hole in the wall, in order to utilize them.
Throughout each area, Dan will encounter monsters and aliens of various kinds. Each area will have a particular set of aliens that serve as that area's primary inhabitant. These aliens are all part of a single evolutionary pattern. They will exist in one of four phases. If an alien in any phase other than the fourth remains on the screen for a long enough period of time, it will begin to evolve to the next phase. It is in Dan's best interests to remove the aliens before they have a chance to evolve and become more dangerous. The boss of each area serves as the ultimate evolutionary form of that area's alien species.
When Dan destroys many of the aliens, they will often leave a blue/green energy coin behind, which count as 10 units of energy. These energy coins have two purposes. First, they serve as money throughout the game. If Dan intends to purchase an item, he must have enough Energy coins in his inventory to pay for it. However, these coins also serve a secondary purpose as the energy required to use many of his alternate weapons. All weapons besides the Heat Knife require energy to use. If Dan does not have enough energy, he will be unable to use many of his weapons. Later in the game, Dan is likely to find red energy coins which provide 20 units of energy.
Throughout each area, Dan will also find many doors in various locations. Behind these doors, Dan will find helpful beings who will provide Dan with information, offer to sell him items, or generally aid him in some fashion. Below is a few examples of the most common inhabitants, but they are not the only ones to be found.
Many of the doors that Dan investigates will contain aliens or old men who provide Dan with valuable information. Paying attention to what they have to say can help you clear an area faster, or discover secrets.
There are a variety of shops in the game. Some shops sell new weapons to Dan, while other shops offer powerful upgrades to weapons Dan already has. Yet another kind of shop is run by women who have useful items to sell.
Dan is very likely to take damage while trying to fight through each area in an effort to find the boss. Inns are available for Dan to stay in and recover all of his lost health. However, he can only stay in each inn once.
Last but not least, you will also encounter a friendly alien who likes to challenge you to a wrestling contest. If you accept his challenge, you must push as rapidly as possible to push the alien to the back of the room. If you win, he'll reward you with 160 units of energy!