The Adventures of MassMouth is the first in a series of three PWADs by Mike "Cyb" Watson. The original version was released on February 23, 2000. This does not run with the latest versions of ZDoom, so the older version 1.22 is needed to play it. In 2003, an updated version was released titled The Adventures of MassMouth Do Over!, fixing bugs and also adding several extra features. This updated version runs properly with version 2.0 of ZDoom.
MassMouth is an alien from the planet Zeeble (a zeebloid). He has a small, green body with a large mouth. An antenna containing a single eye protrudes from his body. He also has two arms and legs. MassMouth resembles the Yorps of the Commander Keen series. MassMouth's preferred weapon is his ray-gun, a weapon which shoots green bolts. The design of MassMouth comes from a Quake II player model created by Stephen "Neuralstasis" Monteith. Massmouth is an employee of The Worm, a rich alien with his own personal moon. The Worm employs MassMouth to bring him various valuable items. MassMouth is very well-paid and has his own personal room.
- Otis : Guards with 100 hit points. They use pistols and drop clips of bullets when killed.
- Barney : Other type of guards. Their pistols are much more powerful than the guns used by the Otis. They have 100 HP.
- Black ops : These are soldiers wearing black uniform and using machine guns. They are more threatening than the standard guards because they have 125 HP instead of 100 and they can kill you quick.
- Stuka Bats : These small bats appear only in the fifth level. They attack by biting you and emit loud and screeching noises. They only have 65 hit points but always attack in groups.
- Shock Trooper : The shock troopers are tall, humanoid aliens who shoot blue fireballs. They only appear in the ninth level and have 800 HP.
- Dwarf Baron : It is a tiny baron of hell that only appears in the fourth level. In that map, MassMouth's objective is to capture (NOT to kill) this creature so it ends up in The Worm's collection. Although it is very small, it is not inoffensive because it throws green plasma balls at the player. It has 250 health points.
- Giant Demon : This monster appears only in the seventh level. At first, it was just a simple Demon but an experiment made it grow in size. It has 600 health points.
- The Voltigore : The final boss. It is a large creature that spits red homing fireballs which are strong enough to kill you in one hit. It spots the player when (s)he picks up the Eye of Mahan. The Voltigore has 10000 hit points, which make it nearly invincible. Upon its death, the end teleporter opens.
Various monsters from Doom also fight MassMouth, albeit only in the fourth level.
Most weapons are reskins of old weapons.
- Fist : Self-explanatory.
- Zippo lighter : Its only purpose is to light the area in the mine level. Can not be used to fight enemies.
- Raygun : Your little raygun. It shoots green plasma projectiles, but it is really weak. Powered by energy ray cells.
- Desert Eagle : A pistol that is actually a reskin of the shotgun. It uses shells so it is good at close combat.
- MP3 : A machine gun which is just like the chaingun.
- Eye of Mahan : It occupies slot 5 and replaces the rocket launcher. It has no purpose, other than being the final item that you have to steal for The Worm. It uses the sprite of the Sapphire Wand from Hexen.
- Plasma rifle : It works the same as its original counterpart, except that the plasma balls are red rather than blue.
- SonicBlaster mine tool : The most powerful weapon in the game. It is found once in the mine level, but a secret has to be solved in order to get it. It emits a very loud noise which has dire consequences on organic structures. Its ammo are cell packs and it is the replacement of the BFG9000. However, it consumes 20 cells per shot, not 40.
- MAP01 : The Worm's Moon
- MAP02 : The Source
- MAP03 : Back At The Moon
- MAP04 : Temple Of The Baron
- MAP05 : Crystal Mines
- MAP06 : The Moon Again
- MAP07 : The Blur Sphere
- MAP08 : The Collection
- MAP09 : The Eye Of Mahan
- MAP10 : Epilogue
Levels 01, 03, 06 and 08 just consist of going into the hall where The Worm is. The final level is just a cutscene level.
The Massmouth games all contain references to Half-Life:
- One of The Worm's unseen servants is called "Freeman", who, in the schedule in the garage, is going to Xen. Gordon Freeman is the protagonist from the Half-Life games who, later in the game is teleported to the dimension Xen.
- The planet "Bahl" is named after Steve Bahl, one of the staff who created Half-Life.
- Many of the enemies, (plus The Worm) are actually enemies and characters from Half-Life: Opposing Force (except they replace, and behave like Doom enemies).
- "The Worm" is based on the Pitworm.
- "Shock Troopers" and "Black Ops" are based on the Half-Life characters of the same names.
- The "Security Guard" is based on Black Mesa security guard Barney Colhoun, first appearing in Half-Life: Blue Shift .
- Massmouth also appears in Action Doom 2 as a graffiti on some walls.