Secret of the Oracle is episode 1 of the Goodbye Galaxy! series.
Secret of the Oracle is the fourth episode in the classic PC series Commander Keen, which pits the young Keen against otherworldly aliens in his dreamlike adventures. It's officially part of of the dual release Commander Keen in Goodbye Galaxy!, which also featured Commander Keen 5: The Armageddon Machine.
Keen assembles a radio and unintentionally intercepts a message from Gnosticus IV about an evil race of aliens attempting to destroy the universe. On arriving at the planet, he discovers that the Keepers of the Oracle have all been kidnapped, and he races around the surface of the planet rescuing all eight of them.
Gameplay is largely unchanged from the previous installments, although the overworld and in-game levels are more detailed and feature more elevators and key hunts. Keen is still armed with his ray gun, which is the only way to kill (most) enemies. He keeps his pogo stick as well, which can be used to reach higher platforms and will also crush certain enemies if you jump on top of them.
The platforming itself is still much less forgiving than a typical console platformer - the player is often required to jump on ledges that are no wider than Commander Keen himself, and many of the flying enemies follow patterns that are extremely difficult to intercept with your ray gun.
Several of the levels are hidden at first glance on the overworld - the mirage level (visible on the above screenshot) appears and disappears at random intervals and can only be entered when it pops into view.
Completed levels are flagged quite literally using the flagpoles beside each entrance.
This game is the source of the legendary Dopefish character, which has cameoed in numerous games from Quake to Sin Episode 1: Emergence. The basic mechanic of Dopefish is to instantly kill Commander Keen by eating him, similar to Big Bertha in Super Mario Bros 3.
Secret of the Oracle also includes a Pong-like minigame on Keen's watch, although the orientation of the game is 90 degrees off from the original.