Mercenary: The Second City is an add-on for the classic game Mercenary that gives the player a new environment and new missions to solve. It was first released for the ZX Spectrum in 1988.
Not a sequel, but a ‘data disk’ add-on that you load through the original Mercenary. The Second City starts off pretty much the same as Mercenary – crashing on a planet (this time the other side of the one you explored in Mercenary, and also a different colour) and having to explore to get on and eventually escape.
The Atari ST‘s power (relative to the Commodore 64) means smoother, faster 3D graphics; more colours (useful, when colours are used to identify rooms and places underground); and more sensitive controls.