This is perhaps the best game that was ever made for the original NES. You are Jason. Your pet frog wandered away from his cage and was mutated by some radioactive material, and then jumped down a hole. You follow him down the hole only to find yourself next to a tank-like vehicle complete with space-suit.
Throughout the game, you are fighting robot machines and mutated radioactive fiends, and at the end of each level you get to add an upgrade to the tank vehicle. By the end of the game, the vehicle can crawl on walls and ceilings, hover in the air, and swim through the water.
This is a very difficult game, but this is what makes it fun. I remember the sense of accomplishment as I finished Stage 3, and then again as I finished the game. After 15 years, I still enjoy playing the game once in a while.