Lassie

The Awakening (17x19)

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Teenage Patty, reluctantly having left the city for a summer in the country with her aunt, has an inauspicious beginning: she's poisoned by arsenic in the well. Her aunt finds the source of the problem in an old mine, but is then trapped when the floor collapses, so Lassie must find Patty, who's just about to leave.