Planetoid D-1 is unlocked after obtaining Planetoid Map D-1 on Planetoid A-3. Planetoid D-1 consists of five rooms. The first room features the Landing Point, as well as three different pathways for the player to take; the left and top right paths can be accessed with the tank, but the right pathway can only be accessed by exiting the tank.
The room to the right of the starting room features two ladders, one on each side of the room; two pools of water, and is a lockdown segment against a Helmet Armadillo. By standing in the lower pool of water and jumping up and down will firing, Jason can defeat the mutant and obtain the SP Up. To exit the room, Jason must jump onto the right ladder, being careful not to touch the spikes below it, climb up to the top platform, and land in the higher pool of water to prevent fall damage; if the player holds left the entire walk back on the top platform, Jason will overshoot the pool of water, thus, the player needs to let go at some point after falling from the ledge.
The room to the left of the starting room features a lockdown segment, where the player has to defeat all of the mutants before being allowed to proceed. After clearing the lockdown segment, the player can continue to a room farther to the left; this room features another, identical lockdown segment, but the room now has flamethrowers, two pairs of them on the ceiling and a pair on the left wall. Beating this lockdown segment gives access to Planetoid Map D-2.
The room to the top right of the starting room features a boss fight against Octa Biby; defeating it gives access to Planetoid Map D-4.
Unlike most planetoids, visiting this one is required in order to progress through the game. By visiting Planetoid D-1, the player can obtain Planetoid Map D-2. By visiting Planetoid D-2, the player can then fight Leibniz, which is required in order for the player to gain access to the Dimensional Tunnel leading to Area F-2, even if Access Key 5 has been obtained from Tesset.
- Helmet Armadillo
- Antibody Cell
- Mutant Attacker
- Mother Brain
- Daughter Brain
- Bomb Dropper
- Biby Larva
- SP Up
- Planetoid Map D-2
- Planetoid Map D-4