The workhorse of the Subterraneans’ attack force, the Bell-Fortress is a gigantic airborne war machine that is impervious to most modern weaponry. The crafts were first utilized against humanity in Antarctica, where their giant Gravity Lens destroyed a series of United Nation ships conducting research on the Indra cylinders.

In retaliation, the UN dispatches a mammoth attack force to the location. Once again, they come in contact with one of the Bell-Fortress’ Gravity Lens, but this time are successful in destroying it. Their victory is short lived, however, as the source of the giant rings then emerges from the clouds above, after destroying a fleet of jets. Out in the open, the Bell-Fortress then releases half a dozen more Gravity Lens, which quickly reflects beams sent from the nearby Indra and decimates the entire front line of the UN retaliatory force.

Fortunately, the Indra's appearance has attracted the attention of another craft, as the undersea battleship Ra emerges to engage the Bell-Fortress. In response, the Indra fires another blast that is then redirected at the Ra by one of the Gravity Lens. However, the beam is deflected by the Ra’s magnetic shield. Unsuccessful in its previous attempt, the enemy fortress then opts for an alternative attack method, recalling the Gravity Lens as it lands on top of the Ra, trapping the ship in its hollow exterior. The war machine then begins to increase the gravity thrust in an effort to crush the Ra. Thankfully, the magnetic shield holds against the attack, as the main cannons are then aimed. The Ra then lowers the shield for approximately 1/3rd of a second, allowing it to discharge its cannons that cause the Bell-Fortress to erupt in a bath of flames.

This victory is short lived, though, as the Subterraneans' force then increases the magnitude of the attack, sending Bell-Fortresses across the world to destroy humanity’s major metropolises. The ships, along with hundreds of Gravity Lens, are successful in rendering their opposition helpless, with the sole exception of the Ra. The undersea battleship, now airborne, answers the attack by decimating the attack forces in an attempt to silence the large Indra cylinders. The Ra is successful in its bid to wipe out the Bell-Fortresses, but is unsuccessful against the Indra’s thick armament, as a new plan to locate the “mother ship” is then put into effect.