When Jack encounters a wandering Little Sister and its fallen Big Daddy , Atlas urges Jack to kill the Little Sister to harvest her ADAM for himself ; Dr. Tenenbaum overhears this and intercepts Jack before he harms the Little Sister , urging him to spare the child , providing him with a plasmid that would force the sea slug out of her body .
Overhearing Atlas ' words , Dr. Tenenbaum intercepts Jack , and urges him to save the Little Sisters instead , giving him a plasmid that will displace the embedded sea slugs in each Sister .