The young pathologist David Coleman (Ben Gazzara) arrives to join a hospital pathology lab. He encounters disorganization and a hostile, cigar-smoking chief, Joe Pearson (Frederic March), who declares his intention to keep working until he dies. Coleman tries to implement a few changes, but his suggestions are overruled.

The film revolves around two cases: possible erythroblastosis in the child of an intern and his wife whose first child died; possible bone cancer in Coleman's girlfriend, student nurse Kathy Hunt (Ina Balin). The infant's problem is misdiagnosed due to Pearson's refusal to order the new Coombs' test recommended by Coleman; the baby nearly dies, alienating the obstetrician (Eddie Albert), a long time friend who now presses for Pearson's dismissal.

Coleman disagrees with Pearson, who thinks that Kathy's bone tumor is malignant, but he opts for professional discretion, defers to the chief, and urges her to have her leg amputated anyway. He discovers that Pearson had been right: the surgery, which he thought unnecessary, has provided her with her only chance of survival. Just as Coleman realizes the enormity of his error, he learns that Pearson has resigned and that he will take over the lab.