Character pages are not supposed to be plot summaries.
Martha Tyler was lived with her grandson Jack Tyler in Fetchborough in the 1970s. She worked in the kitchen at Fetch Priory. She was a practising white witch and could read people's fortunes. By 1977, her husband had died.
In July 1977, a security team was called by Dr Fendelman to guard the priory after the body of a hiker was found in the neighbouring woods. After the leader of this team, David Mitchell, offended her, Martha told Adam Colby and Thea Ransome to tell Fendelman she would return "when the others is gone".
When she returned to her cottage, Jack and Leela found Martha to be in a petrified state. She spoke of something "hungry for [her] soul". Leela fetched the Fourth Doctor and, together, they attempted to revive the old lady by creating a commonplace scenario, talking of tea and fruitcake. The Doctor finally succeeded after he described a method of making fruitcake she disagreed with.
Martha told the Doctor she had the "second sight" and described the being she had seen, with her mind, in the woods. She told the Doctor it didn't have a human shape. The Doctor left and she gave Leela a charm before she followed him. With them gone, Martha told Jack that the being was "a woman".
When Jack returned from scouting out the priory, as the Doctor requested, he found Martha using her tarot cards. Jack told her the Doctor wasn't back, and she replied that one should never trust a man wearing a hat. When her grandson commented that her husband, his grandfather had, she replied that he was a "a wicked old devil". Jack said he wore one, but Martha said that was different, as she had given it to him. She offered him a charm and, despite his disbelief and objections, he agreed to take it, saying it was to please her. Martha then asked for the cartridges from his gun and said she was going to fill them with salt, as that was "the best protection there be". Jack asked if this involved "evil spirits, again". She told him he could laugh, but she knew the old ways. Martha told Jack he was best getting back to the priory, as neither of them would want Fendelman's team "meddling in things they don't understand".
At Fetch Priory, Martha and Jack encountered the Doctor and Leela as a Fendahleen bore down on them. The three humans found their legs rooted. The Doctor took Jack's gun and killed the creature, freeing them. After the Doctor and Leela freed Colby from the cellars, he joined Martha and Jack and blamed the old woman for recent events, Maximillian Stael having said that Martha's visions coming true had led to the making of his plans. Jack told him to watch his mouth after he called Martha a "stupid old witch". Leela also berated Colby, saying he had almost gotten them killed in the cellars.
When the Doctor found himself in need of salt to combat the Fendahl, Martha asked Leela and Jack for their charms and handed them to the Time Lord. They contained rock salt. The Doctor tasked her with collecting as much salt as she could find, of any kind. Having done so, she brought it to Fendelman's lab on a trolley. The Doctor told her and Jack to return to their cottage. There, they sheltered under a table, where Colby joined them after a time, as Fetch Priory was engulfed by the flames of the Doctor's controlled implosion. Having fled the priory themselves, Leela asked the Doctor if it was okay to leave the others, and he told her, "Yes. Probably at Mrs Tyler's now, eating plum cake off her best china." (TV: Image of the Fendahl)