This is an Adobe Flash game produced by Ninjakiwi, the same studio that produces the famous Bloons series. It was released in about 2013. 

It is a 2D zombie shooter RPG game, based around killing zombies and earning better weapons. To that end, there exist a large number of very effective weapon drop hacks in the KongHack community  There is a very weak general "story" to the game, as follows:
"The year is 3104, and a virus outbreak on the trans-federation planet of Thera threatens the very fate of mankind. The only thing stopping this new and terrifying zombie horde from pushing mankind to the brink of existence is the highly skilled and highly weaponized special forces of the SAS."
Each of the maps has some sort of corresponding story element as well. Map selection is limited but fairly diverse. Some of them are more efficient than others in leveling your character. 

Unfortunately, the gameplay can be a little frustrating due to the extremely high graphical demands (lol) of this flash game. Lag is very real, and multiplayer bugs run abound. 

Furthermore, the balance in the game is very very very very non-existent, and regulated only via a wonky RNG system. There are very powerful guns that can be obtained at low levels if you are lucky enough, that are unobtainable at higher levels. Not terribly logical. There are waves of times when your level is just right to get the rare guns, after which you literally won't be able to find them again...without the use of hacks. 

Thankfully, KongHack has a wide variety of hacks to increase your supply of rare weapons. Overall a game with good replayability.