Marielle, a frail and isolated girl recovering from health struggles, lives in a remote mountain house straddling the Swiss-French border. Governed by a strict governess and household staff, her secluded world offers little beyond routine and solitude—until she discovers Memoru, a tiny alien survivor of a crashed spacecraft. When Marielle nears death during a health crisis, Memoru’s tears miraculously revive her, sparking an inseparable bond. Through their clandestine friendship, Marielle gains physical strength and emotional fortitude, collaborating with Memoru to shield his alien community from watchful authorities, their alliance weaving secrecy and trust.

As her health stabilizes, Marielle transitions to boarding school, navigating classmates’ bullying and forging a bond with Oscar, a peer who evolves into a confidant privy to her extraterrestrial secret. Her absent father’s constant travels deepen her early loneliness, yet her bond with Memoru’s group flourishes through cultural exchange: she discovers their otherworldly origins while sharing human customs. The narrative traces her evolution from a sheltered, fragile child to a resilient bridge between species.

In the OVA *Marielle no Hōsekibako*, she narrates past adventures with Memoru, the compilation interweaving episodic highlights through her reflective voice. Framing these events as cherished memories, the OVA underscores her enduring ties to the aliens, though it refrains from expanding her personal growth beyond established arcs.