8 levels, designed to be played solo (although not together). (These are 8 of the 19 in the Infinity version.) A bit linear at first... but good for a bit of carnage, if that's what you're looking for. No real plot on any of 'em.
Six level solo scenario. Nicely done, with some thought put into how to make you flip switches in a specific order without making it look like you're being made to flip switches in a certain order.