Plot

A terrified typist rushes into Mason's office pretending to be a temp. The office of the diamond importers on the floor above has been invaded. Later a partner, George Baxter, is killed, and another partner, Duane Jefferson, is charged.

Perry Mason is drawn into a strange case when an attractive young woman, Patricia Taylor, arrives at his office claiming to be the temp they requested from an agency. She puts in a days work and Della even compliments her on the quality of her output. She suddenly vanishes, however, and they learn that she wasn't from the agency at all. It turns out she was in the offices of diamond merchants on the floor above Perry's and there may have been a theft. When one of the partners in the diamond business is murdered another partner, Duane Jefferson, is charged. Perry defends him and Jefferson claims to have been in the company of a woman but refuses to identify the married woman's name. Nothing is at it seems however and it's left to Perry to sort out the facts with the help of Della Street and Paul Drake.—garykmcd

Walter Lumis (Ben Wright) and Duane Jefferson are partners in the South African Diamond Company, with offices in L.A.'s Brent Building. Lumis and his partner (Alan Marshal) leave the office for the airport to meet their South African partner, George Baxter (Jack Raine). A woman (Joanna Moore) - the "Terrified Typist" of the title - enters the office and starts searching for something. She's caught by Baxter, who has arrived early. She escapes him, and enters Perry's outer office. Gertie (Connie Cezon), the rarely-seen receptionist, assumes this is the typist Della requested from a temp agency, and the woman plays along. In Perry's private office, Paul mentions the police search for a would-be burglar of the SADC offices. Della realizes that the description fits her new typist, who has suddenly left.

Perry goes up to the SADC offices, on the way encountering an attractive woman (Joan Elan). (Much later, he'll learn this is Mrs. Lumis). Mr. Lumis tells Perry that after reporting the attempted burglary, Baxter disappeared. A "wharf rat" named Jack Gillly (Hank Patterson) sees a man dumping a body in the water, recovers it, and reports to Lt. Tragg. The corpse is Baxter, and in a line-up, Gilly points out the man he saw dumping it, identified as Duane Johnson, who quickly becomes Perry's client. As usual, Perry is faced with a client who's less than forthcoming, saying he has an alibi, but because it was a married lady, he won't identify her.

Meanwhile, Paul has tracked down the typist - she's Patricia Taylor, a senator's wife. Perry and Paul interview her, and she admits that when she first met her future husband, she found him pompous and drew some mocking cartoons which she sent to Duane Johnson, whom she thought was a friend. Later, she wrote to get them back and Johnson wrote that he'd hand them over, but at a meeting on the night of Baxter's murder, she's told that the letters could be used against the senator, unless some payment was forthcoming. Perry pressures her to come forward, so she does - to D.A. Burger.

In the People v. Jefferson jury trial, Heinrich (Harold Dyrenforth), a servant at the house Baxter had rented, testifies that he heard Baxter in an argument earlier in the evening of the murder. Perry gets him to admit that people could have come and gone without him knowing, and that he didn't see or hear any actual violence. The defendant takes the stand but still refuses to name his alibi. Patricia testifies she was at the restaurant to discuss the cartoons for about 15 minutes. A photo of the meeting was taken by a restaurant photographer. There's another photo of the defendant there with a different woman. Burger is more interested in bringing out details of the attempted blackmail, including introducing apparently friendly letters from Johnson to Patricia. He also points out that the meeting at the restaurant was short enough that it allowed the defendant time to commit the murder and dump the body. The jury returns a verdict of guilty on the charge of 1st degree murder.

The papers are full of predictions that Perry's going to lose a client to the gas chamber, but he's more preoccupied by that second photo. The woman in it looks familiar, and he finally places her as the one he saw exiting the SADC offices. He correctly guesses it might be Mrs. Lumis, and she's the one his client is protecting. She agrees to testify, but once Perry leaves, we learn that this will require her to stay behind while Mr. Lumis heads to Mexico with a "package". Then Lumis goes to retrieve a man he's been keeping prisoner, but Paul and Perry catch him.

In court, Perry reveals that the kidnapped man he just rescued is the real Duane Jefferson (Steve Carruthers), and the man he's been defending is an imposter, James Kincaid. Perry moves to have the entire case thrown out on a technicality. Some of the evidence Burger had produced (the letters to Patricia) were from the real Jefferson, not Kincaid. So this was false evidence with respect to the defendant, even though Burger had acted in good faith (since he believed that the real Jefferson was on trial). With the entire case thrown into confusion, the judge continues the hearing to a later day.

In Perry's office, he explains how Lumis wanted to steal diamonds from Baxter. When the real Jefferson wouldn't go along, he conspired with Kincaid - who decided to add some blackmail and murder into what Lumis expected to be mere larceny. Because of the identity confusion, Burger is going to have to try the entire case over again, but Perry is sure he'll win in the end. It's clear that Perry feels no obligation at all to Kincaid, because he agreed to defend a man named Duane Jefferson. Perry, Paul, and Della are on their way to dinner with the real Jefferson, a treat by Senator and Mrs. Taylor, who are grateful that Perry didn't introduce Patricia's cartoons in the trial.