Fireboy & Watergirl in The Forest Temple is the first game in the series.
The release date was November 19, 2009.
The main menu.
The level select.
There are 32 levels, consisting of 17 normal levels, 6 simultaneous designed levels, and 9 green/white diamond levels. In a normal level there will be various puzzles to solve, such as levers, pushers, boxes, etc. and a few diamonds to collect, though not necessary to complete the level, it will increase your rating at the end. In simultaneous designed levels, both characters should be moved at the same time symmetrically. Usually, there are not many puzzles and only obstacles. In a green diamond level there is only one diamond that must be collected in order to pass the level, pretty much the same as a normal level but with only one diamond which is needed to complete the level (you can go through the exit doors without getting the green diamond, but you will not pass).
There are two characters, Fireboy and Watergirl. Fireboy can walk in lava but cannot walk in water or green goo. Watergirl can walk in water but cannot walk in lava or green goo.
Fireboy & Watergirl in The Forest Temple Sample Gameplay