Dragon Ball Z, Vol. 15 by Akira Toriyama

Cell has been awakened: a bio-engineered monstrosity designed to become the ultimate weapon, a being that eats whole cities to grow stronger. To stop its rampage, Piccolo challenges Cell while the Super Saiyans undergo unimaginable training in a room where one year passes for every day outside.But Cell's true goal is to merge with Androids #17 and #18, which will increase its strength exponentially. As Cell becomes even stronger, Piccolo has only once chance: destroy #17 and #18 before Cell can merge with them, or no force on earth will stand a chance!