Donald mentions the Spear of Selene in connection with a previous adventure in which he feels that Scrooge acted irresponsibly, with Scrooge retorting that "[he] was not responsible for the Spear of Selene!"
Dewey and Webby forced the Sunchaser to land on Ithaquack, believing it to be the hiding place of the spear and thus where they might find answers about Della's fate. After encountering Selene herself, however, the pair learned that she was unacquainted with any artifact of that name, leaving the true nature of the spear unknown.
According to a blueprint, the Spear is a rocket ship and the blueprint was made on the week Huey, Dewey and Louie were born.
Scrooge later revealed that Della envisioned the Spear as a means of traveling in space, and that Scrooge had built it in secret for her due to Donald's objections and a surprise to Della for the celebrated birth of her sons. Della discovered it and took it on a test flight, only to be caught in a cosmic storm and lost in space. Scrooge made every effort to find her and the Spear, spending a large part of his fortune on constructing replicas of the ship to send after her. Unfortunately, he never found Della or the Spear.
After Magica's shadow crisis has been staved off, the wreckage of the Spear of Selene is seen on the Moon. Della hooks up a tv and finds news footage of McDuck's victory over Magica having saved Duckburg. Although unable to contact them, Della is happy to see her boys.
Throughout the episode years before The Shadow War, Della used the ship as a shelter on the Moon. She had rebuilt the rocket hoping to fly off the Moon on her own. Just when she had found gold from her tooth, the Moon Mite pulls it underground and seemingly cripples the ship with its corrosive saliva. As Della was about to accept her fate, she found gold within the Moon's city of Tranquility in hopes of restoring the Spear of Selene to full capacity.
Using a combination of the spear's remaining parts and gold Della manages to rebuild the spear completely. Due to an idea of Penumbra, Della wanted to modify the spear to fit all the moon Landers and take them to earth. Though she is unable to fulfill this promise as she is forced to leave early due to an irreversible emergency launch initiated by Penumbra, but before she leaves she hands Lunaris the manual of the ship. At the end of the episode Donald comes across the ship, while looking for Della he accidentally initiates a launch, causing the ship to take off once again.
Throughout the episode the spear can be seen leaving the earth's atmosphere where after a journey through space it once again crashes on the moon, due to Donald's inability to pilot the ship.
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