As Fei Chonglin said, their mission this time was to come to Sea City ahead of time and talk to the Base's Chief Leader. In fact, they had traveled for nearly a month before arriving at the Sea City Base. If they hadn't encountered a group of fierce post-apocalyptic world robbers and lost their car to them halfway, they might have arrived long ago.
Post-apocalyptic world robbers were all psychos who would die for food. They would even eat human beings if they got too hungry. Therefore, in order to accomplish their mission, Lin Yu and his teammates ended their battle against the robbers as soon as they could, but let the latter take their car.
However, the five of them didn't know yet that their chief leader had died in a battle not long ago. She had felt that her enemies would soon take action, but had never thought that there was a mole amongst her people. Not only did the mole find out her plan and betray her, but also helped her enemies poison her.