Invasion of the Vorticons
Invasion of the Vorticons is the name given to the first three episodes of Commander Keen. They consist of:
The storyline continues throughout the three episodes, and reaches its conclusion at the end of the third episode. Story elements from this trilogy (such as Spot) can be seen in later episodes.
The Goodbye Galaxy series follows after Invasion of the Vorticons. However, some time before Goodbye Galaxy was released, Keen Dreams silently surfaced, and due to the appearance and behaviour of the engine, Keen Dreams is often considered to sit between Invasion of the Vorticons and Goodbye Galaxy, and is often called Keen 3.5 for this reason.
A hint sheet was released with the registered versions of the games in this series, which contains tips, cheats and other interesting information.