Out-of-work professor Gilbert Jordan Thompson stops a suicidal stranger named Marian Edwards from jumping off a pier and helps her get a job so she can support herself.

Meanwhile, the baby's mother, Marian, arrives a few minutes too late to reclaim her son, and frantically tries to find him, not knowing that the man who saved her life is taking care of her child.

When the girls discover Chum and hear his story, they become his "forty little mothers" and fight for the privilege of taking care of him, also deciding that the professor is not so bad after all.

After staff members see the girls making baby clothes and assume the worst, they alert the strict, no-nonsense headmistress, who finds Chum and fires Professor Thompson for breaking the rules.

Because she thinks Professor Thompson is the husband who deserted Marian and her baby, the headmistress tries to turn his students against him by bringing the mother and child to the classroom to tell them about it.