Return To Mars
The Revenge Of Koop 1: Return To Mars is a Keen 1 mod released by Bloog. It involves aliens known as the Koops who steal Keen's tools and fly with their ship to Mars. Keen follows them in the Bean-with-Bacon Megarocket, but on Mars his pogo and the four ship parts are stolen as well. The mod features four small levels and can be completed in no time. Few graphics are changed, even the titular Koops are "disguised" as Vorticons.
Spoiler warning! This shows the whole levels.