THIS ONE ought to be easy. A third-year Howard University student says that, for no apparent cause, she was yelled at, chased, knocked down and injured and, in part for having resisted all that, arrested a week ago Wednesday evening by a plainclothes D.C. police officer near the intersection of 15th and Euclid streets NW, where she lives.

Chandra Shealey, 19, had arrived from Chicago only the day before, had just registered for classes and was carrying $440 in cash in her purse. She says she heard a man yell as she was crossing Euclid about 7:30 p.m., looked back to see only a seeming civilian in a white car and kept walking. She heard a second yell, she says, looked back again and saw running toward her a man in a T-shirt who grabbed and knocked her down. She says that only when they had been separated by a uniformed officer drawn to the scene did she learn that her out-of-uniform assailant was also a policeman.

Taken to the station house, she was initially charged with a pedestrian violation -- she had allegedly been jaywalking -- and finally with what she says was resisting arrest and police say was disorderly conduct. In the interim, she had had to be taken to D.C. General Hospital for a swollen ankle, diagnosed as sprained. The whole incident kept her until the wee hours of Thursday morning.

The police say they have no comment, other than a) to note that they are conducting the investigation that is standard procedure whenever force is used in an arrest or an arrested party has to be taken to a hospital, and b) to invite Miss Shealey, if she believes herself to have been abused, to file a complaint with the Civilian Complaint Review Board. (A police source also says that the car in the case was not unmarked, implying that Miss Shealey should have understood from that that she was being hailed by, and chased down by, a policeman.)

She says she will complain, but whoa: that's not enough. If events were indeed as she has described, the police department hardly washes its hands of them by invoking standard procedure and inviting her to complain. It is too egregious a tale; the burden of correcting such misconduct, if that is what occurred, surely does not lie with its victim. If the department thinks that events were not as she has described, it likewise has an obligation to say so. But it cannot brush such accusations off.

The veteran arresting officer is named Wayne Simpson. He was on undercover narcotics duty, hardly easy work. If he has a defense, we ought to hear it. If Miss Shealey's account is correct, and no more, he ought not be on the force at all.