The phlogistonite canisters
Phlogistonite was first discovered by Percival Lowell. It is a rare substance, with explosive properties so powerful that it is capable of launching a train-sized bullet into space. This substance was responsible for powering the first space cannon, and thus how the events of Martian Dreams came to pass.
After Andrew Carnegie had finished constructing a second space cannon, the only remaining task was to collect the phlogistonite fuel canisters from the space bullet. When the Avatar's group arrived to retrieve them, however, they found them missing. With the help of Theodore Roosevelt, they determined that Raxachk (still in the body of Rasputin) had stolen them. Upon confronting Raxachk in Argyre, the villain boasted of having perfected a way to move material objects into the Dream-World -- and had done so with the precious phlogistonite. The Avatar had to rescue all the Martians trapped in the Dream-World before retrieving the fuel, or a cataclysm of "epic proportions" could destroy Mars.
Once a way of transferring the consciousness of the trapped Martians into metallic bodies had been devised, using Chsheket as a test subject, these instructions were given to Jack Segal to free the others while the Avatar went to deal with Raxachk. The fiend had fled into the Dream-World, abandoning Rasputin's body, but leaving the Dream Machine functional. The Avatar used the device to seek out Raxachk and, after a final confrontation, was able to safely retrieve the phlogistonite canisters, for the good of Mars and Earth.