Read this sometime in the 80's(actually read it after 1999). A mission from an intricate lab (in the present(?)) back to Medieval times went awry when a soldier took back live munitions, specifically a hand grenade, against scientists desires with disastrous results. Other missions had taken place, and the local inhabitants were hostile. Two of the main characters were a man and a woman.
In Corazon, Dan Baker and his wife are lost and driving through the Northern Arizona desert when they come across a man in his seventies who looks like a priest. They pull over to help him, then take him to a hospital in Gallup, New Mexico. They learn that the man works at ITC and has unexplainable growth abnormalities in his blood vessels. The man dies twenty-four hours later.
In the Dordogne (southwest) portion of France, Archeology Professor Edward Johnston leads a group of relatively young archaeologists-Chris Hughes, Kate Erickson, and André Marek-as they study the fourteenth-century towns of Castelgard and La Roque. Professor Johnston travels to New Mexico because he has reason to believe ITC, their funds provider, is guilty of foul play. During his absence, his students discover several disturbing sights, including the lens to Professor Johnston's glasses and an inexplicable message from him. Chris, Kate, André, and a computer specialist named David Stern are whisked away to ITC Headquarters in New Mexico by the company's vice president John Gordon.
Once there, ITC CEO Robert Doniger informs them that Professor Johnston has traveled to 1357 using their undisclosed quantum technology. The students decide to venture into the past to rescue the professor. Stern chooses to stay behind, realizing that time travel is probably terrible for one's biology.
Immediately when they arrive in 1357, they are attacked by knights. Their ITC guards are murdered, and one activates a grenade before he is fatally wounded, and inadvertently initiates his return, causing the return pad in the present to be severely damaged. Stern and the ITC employees then struggle to repair it so the students can return home.