During the doomsday catastrophe, most of the global human population transforms into mermen. College student Lin Yuan slaughters the bitten campus belle Lin Wan'er on the rooftop, activating the Doomsday Card System to obtain a speedboat and a Tang Dao. Growing stronger through slaughter and card drawing, he successively subdues kendo maiden Luo Qinghan and medical genius Su Yuxi. Su Yuxi dissects a merman and discovers that deep-sea barnacles parasitic on the back of the neck are the mermen's only fatal weakness. Relying on this, Lin Yuan's efficiency surges; he slaughters mutant giant crabs and armored crocodiles, obtains battleship upgrade modules, and upgrades his speedboat into an armored yacht. Afterwards, he charges out of the poison-fog-shrouded university town, slaughters the beast-controlling mutant Chu Xiao and a deep-sea giant octopus, obtains aircraft carrier modules, and ultimately cleaves open the blockade of a ten-thousand-ton sunken ship.